Random chain question

[factstock3]

Chains pop off fairly regularly on bumpy tracks or when getting in the air a bit. Logically speaking, I would think the chain starts to come off at the bottom of the sprocket, so why are the chain guides like prc's mounted on the top part of the chain?
Please let me know if this doesn't make any sense.

 

[factstock3]

Well crap, I just found PRC's actual video, and it is put on the bottom. I had just seen one at the track saturday, and they had it mounted on the top, which confused me. Disregard my question...