Custom moves slightly remix an original special attack for each character in Smash 4. Although some characters hardly benefit from changing their kit, some become downright dangerous just from a few changed properties to a move. Custom moves used to be legal but are now banned at official tournaments.
This video by StylesX2 revisits custom moves and shows exactly why this is the case. They can get ridiculously broken! StylesX2 provides some of the most egregious examples.
Custom moves make Charizard and Ganondorf look viable and stylish, and that should exemplify how much of a boost custom moves can give to characters.
Charizard's Flamethrower can be changed from a short-ranged spray to a fast, aimable projectile that looks more like dragon breath. Its side-B can be changed to an instantaneous burst capable of carrying enemies out of a map's blast line.
Wii Fit Trainer can toss a gigantic ball instead of a smaller one, Samus can release a full charge shot that remains in one place until touched by an enemy, and Ganondorf can angle his down-B horizontally.
The combo potential of these moves are insane to watch and are thankfully kept in casual play. The video itself is nicely edited and serves as a quality reminder that custom moves exist, but thankfully not in tournament.