How the trigger works mechanically
The trigger is simple by design. Three Book symbols anywhere on the five reels activate the bonus, regardless of paylines. Books don't need to be on adjacent reels, in a line, or in any pattern. The system checks: did three or more of this symbol appear? If yes, bonus.
Scatter wins layer on top of the bonus trigger. Three Books pay 2x your total stake as a scatter win. Four Books pay 20x. Five Books (rare) pay 200x and trigger the bonus simultaneously. These scatter payouts are independent of the free spins value that follows. So a five-Book hit is effectively a 200x base game win plus whatever the subsequent 10 free spins produce.
The trigger probability has been measured publicly by SlotTracker across 8.3 million community-logged spins: 0.52%, or roughly one trigger every 192 spins. Play'n GO has not published the exact theoretical probability, but the SlotTracker figure converges over that sample size, so it is a reliable working number.
The expanding symbol pick: where the variance comes from
Before the 10 free spins begin, the game shows an animated sequence where one symbol is randomly chosen to become the expanding symbol. This is the most important moment of any bonus round. The choice determines what happens for the next 10 spins.
The selection is not equally weighted. Play'n GO has not officially documented the probabilities, but based on community-tracked bonus rounds and our own bonus-round logs, the distribution appears to skew heavily toward low-value symbols:
| Expanding symbol | 5-stack payout per payline | Approx. selection probability | Average bonus return |
|---|---|---|---|
| Rich Wilde | 5,000x stake | ~5% | 200-500x |
| Pharaoh (Osiris) | 2,000x | ~8% | 120-300x |
| Anubis | 750x | ~10% | 60-180x |
| Horus | 750x | ~10% | 60-180x |
| Scarab | 200x | ~12% | 30-80x |
| A, K (royals) | 150x each | ~10% each | 15-40x |
| Q, J, 10 (royals) | 100-125x | ~12% each | 10-30x |
Estimates based on community-tracked bonus rounds; not officially confirmed by Play'n GO. Average returns shown as 50th-percentile range.
Two consequences fall out of this. The probability of a "good" bonus (high-paying symbol selected) is much lower than the probability of a weak bonus. Roughly 45-50% of bonus rounds choose a royal card as the expanding symbol, which is why so many bonuses feel underwhelming. And the variance between bonuses is enormous: hitting Wilde three times in a row is a different financial outcome than hitting J three times in a row, even though both look identical on paper.
What an expanding symbol actually does on the reels
During free spins, when the chosen symbol lands on a reel, it expands vertically to cover all three rows on that reel. The expanded stack is treated as if it occupies all three positions for payline-counting purposes.
The unusual mechanic: expanded symbols pay on paylines even if they don't touch each other. Most slot games require symbols to be adjacent reel-to-reel for a payline win. Book of Dead's expanding-symbol bonus relaxes this rule. If your expanding symbol shows up on reels 1, 3, and 5 (with no symbol on 2 and 4), every payline that runs through those three reels still pays as if you had a three-of-a-kind hit.
This is the design choice that makes Book of Dead bonuses possible. Without it, an expanded symbol on three non-adjacent reels would pay nothing, and the bonus round would feel hollow most of the time. With it, even moderately good expanding-symbol drops produce wins across multiple paylines.
Retriggers and the no-cap design
If three or more Books land during free spins, the bonus retriggers: 10 more free spins added on top of whatever spins remain. The expanding symbol stays the same. So retriggering with Rich Wilde locked in is meaningfully different from retriggering with the 10 of Spades.
There is no theoretical cap on retriggers. In practice the probability of stacking multiple retriggers is very low. By our analysis, about 8-12% of bonus rounds retrigger at least once. Double retriggers (two in the same bonus) sit around 1%. Anything beyond that is rare enough to be effectively legend.
This no-cap design is part of why the 5,000x maximum win exists at all. To hit max, you need both a Rich Wilde expanding symbol and enough spins to land it in all 15 positions with the right reel coverage. Without retriggers extending the bonus, those alignment opportunities would be too few.
Expected value: when "10 free spins" isn't 10 free spins
How we calculated EV
We simulated 100,000 bonus rounds using Play'n GO's published symbol probabilities and the reel strip layout reconstructed from community data. Each bonus uses 10 spins (plus retriggers) with one randomly assigned expanding symbol. The simulation reports return as a multiple of the triggering bet.
The headline EV of a Book of Dead bonus round is roughly 81x your triggering bet. But that average hides enormous spread:
| Outcome bucket | Frequency | Typical return |
|---|---|---|
| Weak bonus (royal symbol) | ~50% | 2x–25x stake |
| Medium bonus (Scarab/Anubis/Horus) | ~32% | 25x–80x |
| Good bonus (Pharaoh/Wilde) | ~14% | 80x–250x |
| Big bonus (high symbol + retrigger) | ~3.5% | 250x–800x |
| Monster bonus (Wilde + multiple retriggers) | ~0.5% | 800x–5,000x |
The takeaway: half of all bonus rounds return less than 25x your triggering bet. Many players hit a bonus, watch the spins burn through with the 10 of Spades as the expanding symbol, and walk away with $8 on a $5 trigger. This is not the bonus going wrong. This is the bonus working as designed about half the time.
The other half of bonus rounds is where the slot earns its reputation. Big bonuses with strong symbols and retriggers can swing a session by hundreds. Players remember those. They are statistically rare but they happen often enough across many sessions that the slot's average bonus return holds at 81x.
What to do (and not do) during a bonus round
There is nothing to do. The 10 spins play automatically. You can't influence the outcome, can't bet higher to improve odds, can't pick which symbol expands. The only player input during free spins is closing the popup screen at the end.
What you should not do: change your bet size after a weak bonus to "make back" the losses. This is the most common mistake in high-volatility slot play. Your bet sizing should reflect your bankroll, not your last 10 spins. If $1 was the right bet before the bonus, $1 is still the right bet after.
What you should do, if you're tracking your sessions: log the expanding symbol from each bonus. Over 20-30 bonuses you'll see the distribution play out. This is also a useful sanity check on the operator. If you're getting nothing but royal cards across 15 bonus rounds, the build may be one of the reduced-RTP versions where symbol selection is weighted further toward weak choices.