TwiddleMyFiddle: They never animate the back for some reason too with the movement, it makes it look kinda stiff..

ricoaio: Is it just me or her body is kinda weirdly disproportional in some areas. Like a Rob Liefeld drawing

Harveytherabbit: It's secondary motion. Animators early on know secondary motion in terms of limbs since we see it with punches and such, but sometimes it takes a bit of experience and pointing it out before an animator can perceive secondary animation on a global scale, like him popping against her backside should compress the spine and causing minute corrections there making her arch her back slightly. The more visual elements are the breasts, so secondary motion there is something every teenage boy knows instinctively, but the jiggle is directly related to the spine compression caused by the hip hit since the force is projected into them as non-muscle-driven extremities. So breast jiggle is the biggest visuals for female movement because they have no muscles to hold them back and therefore tend to be the last thing moving.