A semi flip is basically a quarter kicklip follow by half way during the flip a front side varial kickflip then a quarter heelflip.
thumb|300px|left A quarter Kickflip followed by a late backfoot frontside varial one and a quarter heelflip. So the board flips two ways (1/4 kickflip, then 1 1/4 Heelflip) and also does a varial. It's called a semi flip because the flip is split up into two halves.
You need to break it down into bits and put it together in your mind bit by bit, but I promise it makes sense eventually...