Have you ever tried moving the models around lol? Like grabbing their arms to change their position, or grabbing their head, or moving their waist around. Literally it is the easiest and most blatant thing to reproduce in the game's physics whenever there is any collision between the character models when it isn't the penis and black hole orifices that defy conservation of matter.
For instance legs moving and flapping silly, just go into the game, very first pose called "kneeling still", turn the girl's head to face the guy, middle mouse click on the guy's waist so the cock auto aligns. Then move the girl's head so it auto plays. You can see her waist and legs begin to move by themselves instead of the upper torso just pivoting.
Or go into flatiron position simply make the male body smaller, move it around a bit you will see the spasms. Or Facial Ground, middle mouse click the waste and see the girl bend backwards almost instantly, you can change male body size and move it around to see the different spasms.
It has nothing to do with computer processing power and but rather the how game physics (collision ) is set up as I stated.
Also here is a
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to show you how easy it is.