Shortbow

From SlimeWiki

The Bow series of serves are invincible (although this one will need a few tries) serves that impress. This replay shows the serve being not quite perfect, as the ball ends up too far from the net, but all of the serves that end like this (with a tail grab into a flip) have differing results. And I am NOT doing it again (actually, it isn't that hard, besides the second-last hit backwards and the tail grab flip)! It needs to be Unstoppable Tested. Remember to try the serve until you can always do it perfectly (with a perfect flip, unlike this replay) before you test it!

For an even shorter version, make the second-last backwards hit longer and just do the flip off there. I've done it, but I was so amazed I forgot to press space!