In this instance, i'm planning for Quebra to stop only himself from smacking his pecs against the shield - he is barely over half of Rook's full weight, so not much of a chance to overpower the merc even with his souped up calves - but it will end up with him heavily leaning his weight into the shield. Not pushing back though, he'd just be resting up against it.

A knee strike, to be precise, as to let him brace the impact with three limbs instead of two and better stop the momentum of his lower body.