Butter, this is hip extension. First one his hips are flexed. Torso closer to femurs. 2nd one he is fully extended. Torso pelvis femurs in line. So going from pic 1 to pic 2 is extension. The opposite movement would be flexion.
Look at How far his torso moves rearward/up in a very short amount of time while also moving a very large mass.
Look at How far his torso moves rearward/up in a very short amount of time while also moving a very large mass.