A slam stop lets you halt the reel animation early by pressing the spin button or tapping the screen during a spin. It does not change the result, which the RNG has already fixed; it only shows it sooner.
Because it speeds up play and can give a false sense of control, the Gambling Commission banned slam stops on GB-licensed online slots from 31 October 2021, as part of the same game design rules that removed autoplay and turbo spin.
On older land-based machines, similar "skill stop" buttons existed. On RNG games they never affected the outcome, which is decided before the reels begin to move.