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How bonus round mechanics are created
The bonus round is the culmination of the slot: the peak of emotion, where the game shows character and math shows discipline. A good bonus is recognizable, transparent and balanced: it supports target RTP and volatility without breaking the economy and readability. Below is a structured pipeline: from goal setting to release and live optimizations.
1) Why you need a bonus: goals and product KPIs
Emotional curve: "expectation → growth → reward → discharge."
Retention driver: the player returns "to finish the game before the bonus" - it is important not to overheat the wait.
Differentiation: mechanics - USP slot in the showcase.
Monetary metrics: contribution of bonuses to RTP, variance control, average time to first bonus (T2FB), share of retriggers.
UX/KPI: Hit Rate (any win), Bonus Frequency, Avg Bonus Win, p95 "dry" series before bonus.
2) Typologies of bonus mechanics (work card)
1. Free Spins
Classics: free spins with modifiers - multipliers, sticky/expanding wilds, locked reels, cascades.
2. Pick & Win
Selection from closed objects: instant prizes, multipliers, "traps," stairs with risk.
3. Hold & Spin / Respin
"Coin" mode with fixed cells, a try counter, a collection of jackpots (Mini/Major/Grand).
4. Book/Expanding Symbol
The random character expands to the entire reel/field, pays like scatter-line.
5. Clusters & Cascades
Cluster wins, new symbols, growing multipliers for chains.
6. Wheel / Trail / Map
Wheel of Awards; track with steps, risk levels, "boosts" and "dummies."- 7. Feature Store/Buy Bonus (if allowed)
Purchase of entry; requires a separate RTP and compliance balance.
3) Math: How the bonus "holds"
3. 1 Basic values
RTP_total = RTP_base + RTP_bonus (+ RTP_jackpot)
Bonus Frequency (BF): 1/' N 'spins.
Expected Bonus Payout (EBP): Average bonus win in xBet.
Contribution_bonus = BF × EBP/( average rate in xBet).
3. 2 Volatility Management
More often, but weaker: high BF, low EBP → "soft" bonuses.
Less common, but fatter: low BF, high EBP → high volatility.
Tails: Limited to multiplier/step caps, probabilistic "stops" on tracks.
3. 3 Balance of RTP pools
Distribute RTP percentages between base game and bonus (and jackpot, if any).
Avoid "double counting" (for example, multipliers acting both in the base and in the bonus).
4) Triggers and retriggers
Scatter trigger: X characters in any place (transparent to the player).
Symbolic combos: on lines/clusters (better to visually explain).
Counters/collections: filling in the scale (you need an explicit progress bar).
Retriggers: Control frequency and "strength" (extra backs/levels) - otherwise the distribution tail will swell.
Best practice: "soft blind" - several ways of entry (main + rare "supertrigger") for emotional variability without breaking the economy.
5) Design of specific mechanics
5. 1 Free Spins with multiplier
Levers: basic multiplier, its growth in cascades, sticky-wild frequency.
Anti-snowball: multiplier rollback probability, X ceiling, cascade constraint.
5. 2 Hold & Spin (coins)
Grid and counter: 3-4 "lives," resetting the timer with a new coin.
Rarity of cells: table of weights from mini-caches to "Grand."
Combinatorics: "bundles" (row/col/shape) give boosts → do not forget mouthguards.
Option: add "perks" (field extension, doubles), but dose the frequency.
5. 3 Pick & Win (risk ladders)
Pocket Equivalence: The expected value of the cards selected must match the EBP target.
Anti-frustration: limit "dummies," enter "rescue" (consolidation of small winnings).
5. 4 Wheel/Trail
Segments/Steps: Set probabilities so that large prizes are rare but achievable.
Drama: dose the visual "almost hit" so as not to cross the line of manipulation.
5. 5 "Book" extensions
Symbol selection: premium/base equilibrium by probabilities.
Expansion: compensation for "dead" backs in the bonus - do not overestimate the retrigger chances.
6) Prototyping and simulations
Prototype (spreadsheets + scripts): simulate 10 ^ 7-10 ^ 8 spins: BF, EBP, tails, series length.
Stability: Check the sensitivity to small scale edits (robustness).
Heatmaps: Winnings distribution, p95/p99, dry series graph before bonus.
Target tolerances are ± 0. 2–0. 3 p. p. by RTP bonus pool, control p99 "max exposure."
7) Economy and game loop
Time to Bonus (T2FB): Controls emotional temporality.
Average bonus duration: affects the "quality of the minute" (do not delay without action).
Synchronization with the base game: the base must "feed the expectation" (micro-plays/teasers) without zeroing the variance.
8) UX, readability and sound
Entry threshold: trigger animation - short and clear, does not block control.
State machine: status "trigger → choice → game → result → return."
Informativity: spin counter, multiplier, remaining "lives," track progress.
Sound: signals "almost success," winning phrases, "critical" sounds at the last chances.
Accessibility: contrast, subtitles, "quiet mode," vibration tips (mobile).
9) Anti-abuse and honesty
RNG and invariants: RNG is separated from graphics, solutions - to animations.
You cannot "knock out" the bonus with behavior: auto-clicks/restarts do not give edge.
Buy Bonus (if any): separate RTP pool, compliance by jurisdiction (often prohibited).
Transparency: paytable, retrigger probability, multiplier ranges - without "small print."
10) QA and Certification
Math-QA: reconciliation of sims with the RTP standard; frequencies/tails in tolerances.
Deterministic sides: reproduction of rare scenarios (jackpots/max tails).
Load: long sessions, memory leaks, frame misses.
Certification: reports on RNG/mathematics/logs; Fix the scale/table versions.
Regulation: limits on auto-backs/speed, buy-feature, minimum RTP.
11) Localization and cultural codes
Symbolism and metaphors: take care of cultural contexts (religious, historical).
Numbers and formats: local currencies, delimiters, word lengths in UI.
Rule texts: simple, with examples of bonus calculation.
12) A/B and post-release optimization
What to test: input frequency vs bonus strength; number of retryggers; bonus length; type of modifiers.
Guardrails: crash-rate, complaints, NPS, p95 "dry" batches, ARPDAU/GGR stability.
Fine-tuning without changing mathematics: visual pace, hints, sound, duration of animations.
Retention effect: compare cohorts of D1/D7/D30, take into account seasonality.
13) Examples of design templates (simplified)
Template A: "Cascading Freespins with a Growing X"
BF ≈ 1/180, base X = 1, + 1 for each cascade chain, cap X = 20.
1/20 Retigger, + 5 spins.
Target: average duration of 8-12 spins, EBP ≈ 35-45x, p99 ≤ 800x.
Template B: "Hold & Spin with Perks"
Field 5 × 4, start 3 "life," mini-coins 1-5x, rare 50/100/250x.
Perks with rarity R: row expansion, doubling of extreme cells, magnet for neighboring ones.
Target: EBP ≈ 25-35x, cap per total factor ≤ 2500x, frequency 1/210.
Template C: "Wheel + Track"
Wheel for entry: 70% - small prizes, 25% - medium, 5% - entry to the track.
Track 8 steps, 2 "dummies," 1 "save," final super prize 1000x with probability 0. 05%.
Objective: emotional variability without RTP bias; p95 series before super prize - controlled.
14) Producer/Math/Engineering checklist
Producer
- Clear USP bonus and its role in the showcase
- Objectives: RTP contribution, BF, EBP, volatility, T2FB
- Plan A/B and guardrails
Mathematician
- Weight tables, simulations ≥10^7 -10 ^ 8 spins
- Tails/Mouthguards, Retryggers, Sensitivity
- Certification Reports
Engineering/Client
- State machines, deterministic seats for QA
- Performance, animation timing, offline stability
- Rebate Event Logs and Telemetry
UX/Art/Sound
- Readable counters and progress
- Chance/Climax/Discharge sound markers
- Availability Settings and Quiet Mode
Compliance
- Compliance with local rules (auto-spins, buy, minimum RTP)
- Transparent rules and sample calculations
- Privacy-by-Design for Telemetry
Creating a bonus round is emotion engineering expressed in probabilities. You win when:
1. the mechanics are clear and unique, 2. mathematics calibrated (BF/EBP/tails in tolerances), 3. UX and sound support drama without "dark patterns," 4. the slot economy remains stable, 5. QA/certification and post-release analytics close the cycle.
So the bonus becomes not a "lottery for the sake of the lottery," but a strong tool for retention and recognition, which is equally loved by players, shops and operators.