A handful of characters change their form when using certain skills. The earliest you can obtain this, as it was for me, is during the tutorial fight on Wano Island. When facing off against Kaido, Luffy will opt for using Gear Four and enable the trophy for you. If this does not appear, then as you progress through the game, characters like Luffy will obtain a Form Change skill. It will look similar to a Full-Force Burst symbol in Special Move setup, but it will have a different set of skills mapped when in that form. Luffy has at least two, as does playable characters Big Mom, Kaido, and a few Logia-type users too.