Yes, of course it has something to do with torque. LFS just slips the clutch automatically for you. Once the torque is greater then the friction keeping the brakes locked, the car moves forward, and LFS disengauges the handbrake.
Not of course. I just took the MRT out with gearing as tall as possible, and the handbrake still disengaged--and that was below 1000 RPM with no boost and 50% air intake restriction.
I tried the same with normal gears, floored it with the handbrake on and the car wouldn't budge. Let go of the gas and then the handbrake, the handbrake disengages.
Once the torque is greater then the friction keeping the brakes locked, the car moves forward, and LFS disenguages the handbrake. But, the handbrake force was greater then the torque output, so the car didn't move. That is the only relation between the torque of the engine and when the handbrake disenguages.
Point 1: The force doesn't need to be greater than the friction keeping the brakes locked to make LFS disengage the handbrake (this is easily proved by trying to accelerate from 1000 RPM with the handbrake held on manually; the car will not move, and yet the handbrake disengages with the throttle closed if the handbrake is not held manually).
Point 2: The car doesn't have to start moving before the handbrake is automatically disengaged (this can be proved by using the tallest gear possible--the car will not even budge with the handbrake off while idling).
Bear in mind that this is a bug that is only seen (by me) in the few select cars previously mentioned...
[with automatic clutch off]
Example: XFG, XRR, FXR, &c., normal first gear, parking brake on, you can even feather the throttle a little and push the car forward a tad, but the brake will still be on. This is normal.
Example: MRT, FZ5, FZR &c., gear as tall as possible, max intake restriction, < 1000 RPM... brake will not stay on. This is a bug!
Edit: This thread shoulda been locked after Highsider9's post...
I found out a little trick for this hold your break then go off LFS (windows key) then when you come back on you cant move your car what so ever with accelerator you have to touch the brake and then it will let you go.