This happens when you go to sit on the toliet, and as you are making the motion to move down you let a fart out. A little gas to help you bend down. When you sit down on the chair/toilet your face is now at the same level as where your ass was. You then smell the torrent of farts that you let out. Hence giving yourself the ass waft.
Man, I totally ass wafted myself just a minute ago in the bathroom.