Running flat is a term cribbed from the horse world. It means dropping the head down and running too fast to leap.
Many, many disc dogs run flat and cannot leap as a result of it.
Even Michael Jordan can’t jump when running to first base, he’s running too fast.
Proper leaping requires a slower speed so collection can happen. Dogs that run flat are running too fast to properly collect and launch themselves into the air.
Dogs that run flat often wind up over-pursuing and leaping under discs.