Full explanation
Mystery symbols land stacked on reels 2 to 5. When a revealed symbol is part of a paying line, it turns sticky and stays put while the rest of the grid respins. So the more sticky symbols you are sitting on, the more of the grid is already "solved" before the next spin even starts.
That is the whole mechanic in one sentence: each locked symbol removes one source of randomness. With one sticky symbol you need the open reels to do most of the work. With four or five locked in, you only need a small piece to fall into place to keep the chain alive.
Hacksaw does not publish the per-spin numbers, and anyone who quotes you an exact percentage is guessing. What we can say from the design is the direction: probability of a continued respin rises with each added sticky symbol, but not in a clean straight line. The first couple of stickies help the most. After that you hit diminishing returns because a chain still ends the moment a spin produces no win or a Mystery Reel resolves the round.
Practically, this is why a respin chain feels like it either fizzles fast or snowballs. Once three or four symbols stick, the math quietly tilts in your favour for a few spins. The honest takeaway: you cannot force it, but a stack that builds early is the run worth paying attention to.
A practical way to read a chain
Watch the first two respins. If a stack of three or more symbols locks early, the chain has a real chance to build, because each subsequent spin only needs a small contribution to survive. If you are still on a single sticky symbol after two respins, the chain is far more likely to die on the next dead spin.
You cannot act on this in any way that changes the result, since the outcome is already set by the RNG. But it does explain why some spins feel like they have momentum and others feel doomed from the start. The momentum is real in the math; your ability to influence it is not.
The short version for a normal session: more sticky symbols means a better chance the next respin survives, but the feature still ends the moment a spin adds nothing. Enjoy the long chains when they come and do not read meaning into the short ones. Both are the same machine doing the same thing.