What is HALF SPEED VIDEO?

Simply put, it's video that is playing at half its original recorded speed of 30 frames per second. (You did know that Video plays at 30 frames a second, right?)

Isn't that just the same as playing it at 15 frames per second?

No, not at all, but to explain why you'll have to first understand that within a single video image (when recorded by most VCRs and CamCorders) there are actually two separate images interlaced (stitched) together.  These interlaced images are called fields and each field is actually a unique picture. Sometimes the difference is minor, other times it's significant - it depends on how much movement there is in on the screen.

Here's an example

Note in how clear the rocket looks but the ground and buildings are fuzzy and have little lines (fields) around their edges.

Here's the same image with the first of the two fields removed. The second field (not shown) is 1/60th of a second later into the rocket's flight. By taking the video from the Vidroc and converting each field into a frame (Half Speed), you get to see more details in the flight.  This was especially beneficial when studying the ejection charge sequence of flight #3.