Charizard’s Rock Smash special in Super Smash Bros. for Wii U already has a lot going for it: armor, loads of damage, and now, thanks to this technique, the option of forward momentum to make it a threat over much longer ranges. This helps to cover the special move’s lack of start-up speed and effective distance by getting Charizard where it needs to be as the move is being activated.
This added forward momentum comes from canceling a jump into the special move and then rolling the stick forward. The timing between the jump and the special input aren’t quite as tight as plinking moves in Street Fighter IV, but that’s a good way to think of how the button presses should occur.
Source: My Smash Corner via Smash Boards