High alpha flight is when you're past the critical angle of attack, or flying in a stall. That term has been applied to Knife Edge as well, but I'm not sure I understand that.

Harriers, elevators, slow KE's, and rolling harriers are high alpha manuevers.

The Sledge does it better than the Su Do by not rocking it's wings all over the place like the Top Cap and Su Do will. That is, you can still use some mixed in spoilerons to do it with the others, but that adds speed to things like harriers. Being able to do it without any mixes is a special thing. There aren't too many small airplanes that can do it...Luckily, the Sledge is one of them!

So high alpha can only be done with certain airplanes by certain pilots!