It works on a lot of people, but Marth has been one of the best.
Basically, if you have a fully charged gyro and fall with them off the edge, you can shoot it at the right time to prevent them from getting back.
Snake always gets hit, but he just up+b's again and gets back most of the time. Besides, you never get to do it because the only time you can do this is if you hit them far enough away to make them have to use their second jump (you jump with them to continue falling at their speed if you need to), and Snake's always recover high in that circumstance.
The best part about it isn't hitting with teh gyro though. That is easily dodged by air dodging and fast falling. The problem is that ROB DIs towards you and just fairs to knock you back and then can pressure as he will.