This is just an edge hop.
1) grab the edge
2) tap backwards to let go of the edge
3) use your air jump forwards asap
4) use an attack (or anything really, like waveland, airdodge, nothing, etc.)
It actually took me quite a while to master this one, but I believe it is extremely important since it is a major part of edge guarding.
Some other important notes for when you master the edge hop:
- When you first grab the edge, you gain some invincibility frames. Every character does. So doing the ledge hop right away means you will carry these invincibility frames. That's why in the video link the captain falcon does not get hit at all.
- The falcon in the video got a "reverse knee". You can do this by hitting with the back of the hitbox of the knee. It's useful for any time you want to knee them the opposite direction (ie not just for this knee). The back of the hitbox is basically in captain falcon's crotch/ass. Try to have the enemy there when the knee hits.