Their footprints show that, although their hindlimbs left deeper footprints, indicating that the rear limbs were carrying most of the weight, they also left shallow handprints.
The skeleton wasn't complete, but there were enough parts of the arms, legs, and tail to know that the forelimbs of this dinosaur were shorter than the hindlimbs.
So when they jump, they get a little bit of force off the hindlimbs, and they get most of the force from the forelimbs, because the wings are bigger than the legs, it's a flying animal.