Jeez, he play demoman much? Those are some good airshots. I take it the 1-handed sticky launcher is a skin.
About 3 months ago i saw a similar soldier video. But still this is way harder as a demoman.
it's all about knowing the angle of the nades that you shoot and most of all, prediction.
it's basic geometry, you just have to do the math in your head really really quickly, and somehow instantaneously do that in the game... simple! lol