So my E60 LCi 530d had the hold function (no special button to press or anything), just release foot from brake and you had circa 3 seconds I think before it would let go and start to roll back. The M5 kind of has it too, but not as great and actually tends to pull you up the hill rather than hold or release back down. I guess that's down to the immense supercar beating BHP and Torque