A frame is just the way speed/time is measured in fighting games. VF4 runs at 60 frames per second. So if I have an attack that executes in 20 frames, that means it takes 1/3 of a second to come out. In the case of Akira's knee, you have to hit K+G, but let off of the G button in 1 frame (1/60th of a second; the shortest amount of time recognized by the game). If you hold the G button for 2 or more frames, the move won't come out. You can check how many frames you're holding a button by using show input in training mode, type B I believe.