From Bulbapedia, the community-driven Pokémon encyclopedia.
The Rollout storage glitch is a glitch present in the core series Generation VII and VIII games. It was patched out in revision 1.3.1 of Pokémon Sword and Shield.
It allows the damage multiplier of Rollout or Ice Ball to be applied to other moves.
Performing the glitch
If a Pokémon uses either of the moves Rollout or Ice Ball against a Mimikyu with its Disguise intact or an Eiscue in Ice Face mode, the turn number of that move will be incremented, but the damage multiplier will not be (as a Disguise-breaking hit skips the entire process of calculating damage). If the Rollout or Ice Ball then runs to its full 5 turns successfully, the last hit is considered turn 5 but only receives the damage boost of turn 4 (usually ×8). The damage boost that should normally go with turn 5 is then stored for potential use with a different move.
After Rollout or Ice Ball has completed its fifth turn, the Pokémon that used it is no longer locked into any move, and can use any number of intervening status moves. Once it uses a move that calls the damage formula, no matter what that move is, it will receive the damage boost associated with Rollout turn 5: ×16 if it hadn't previously used Defense Curl, and ×32 if it had. If that move is Rollout or Ice Ball again, the user will not be locked into it for any duration.
If the original Rollout or Ice Ball misses or fails for any other reason before it runs to its natural completion, any stored multiplier that may have existed is correctly erased as usual, even if that multiplier managed to become separated from the move's nominal turn count thus far.
If two or more Mimikyu each switch in to have their disguises broken during the duration of a single Rollout, or if Eiscue regains its Ice Face by using Hail, the turn counters can become separated by a margin greater than one. For example, if the Rollout takes the disguises of three separate Mimikyu along the way, then the latest multiplier that ever gets calculated is the turn-2 multiplier (×2). Afterwards, the next damaging attack that Pokémon uses will receive a ×4 multiplier, the attack after that will receive ×8, and the third subsequent attack will receive ×16, after which the game is satisfied that the Rollout is complete in all aspects, and further moves revert back to normal, with no latent multipliers being stored.
If a Pokémon has a stored Rollout multiplier available, but uses a fixed-damage move such as Seismic Toss that doesn't call the damage formula, that move proceeds as normal and the multiplier remains undisturbed, continuing to be stored for future use by a different move.
If the move on which a stored Rollout multiplier gets consumed is a move with variable power, such as Return, Gyro Ball, Low Kick, or Fling, the game will apply that multiplier to the move's raw power as stored in the data table, in lieu of performing any of the normal computation as to what that move's base power should be. In the case of most moves with variable power, including all four mentioned, their raw power is 1, despite some of them using a formula that's incapable of ever producing a power calculation of 1 any other way. Stored Power and Power Trip use 20, Rollout and Ice Ball themselves use 30, and Eruption and Water Spout use 150.
If the move on which a stored Rollout multiplier gets consumed is a multi-target move, for these purposes it's treated as multiple moves in succession, with damage computed separately for each target. If the multi-target move hits the user's own teammate (such as Earthquake or Explosion), damage against that target is computed first. After that, or for a move such as Rock Slide that doesn't attempt to target the teammate at all, or if it does but they were immune to the move, consider the hit against the opponent on the right side (the opponent's left side), then the one on the left side. For each of those targets that successfully gets hit by the move, consume the next lingering Rollout multiplier if there's still one available, otherwise the attack simply deals normal damage against that opponent as if Rollout had never been used at all.