You don't really need to worry about that. Think about why the half squat makes the jump better. The muscles that perform the jump, mostly the quads, cannot generate any more useful force once your feet leave the ground. That's why it's essential for the force to be as strong as possible during the upward phase before your feet leave the ground, and squatting gives your quads more time to ramp up its force so that when your body starts going upwards they're already almost fully engaged. That's not as much of an issue in TT, as your arm will always have an anchor to exert force from. The only time your arm can't generate more force is when the ball comes at you too fast, in which case you certainly won't have time to delay the straightening of your arm.
You've done regular squats, right? Does doing a bit of a "dip" right before you push up make the squat easier? Heck no! Think of a TT loop more like a soccer kick. The relevant muscles are already at their maximum "counter" position (i.e extended thigh and flexed knee) near the beginning of the forward swing. Of note, just like TT, the maximum "counter" position isn't achieved at the very start of the forward swing, merely near the start of it, because the action of the forward swing actually helps achieve the maximum "counter" position just like how the start of a TT forward swing naturally straightens out the elbow.
The mechanic is the conservation of momentum. Since there's a combination of linear and rotational movement in a TT swing, both linear momentum and angular momentum come into play. You go from a big mass moving (basically your whole body) to a small mass moving (just your forearm/hand/racket), a big radius rotation (centered at your body) to a small radius rotation (centered at your elbow), and the still moving object (your forearm/hand/racket) will necessarily have to increase its velocity in order for momentum to be conserved.
It's the same for achieving a spinnier service. Stop everything from moving except your wrist shortly before the impact and your racket will move at a much faster speed.
Wrong, that's only if you want maximum speed, like flat hitting. You want to accelerate through the impact if you're looping. This prolongs dwell time, which increases control and spin. You don't need maximum acceleration, of course, but you want to be accelerating when looping. The balance between maximum acceleration (occurs at the beginning of the swing) and maximum speed (second half of the swing) depends on the shot.
For example, have you ever tried to loop a half-long ball that barely falls off the table wide to your FH side near the net? You need maximum control and spin to land this delicate shot, especially if you don't want it to be high and easily countered. The way to do is to loop the ball at very near the beginning of your swing. This is the same as when you roll a ball off the table and try to loop it. Try doing it with a big backswing. You can't. You need to loop it at near the beginning of your forward swing. I think you understand some Chinese, right? Maybe try this video to see the practical application of some of my points.
