If there is exhaust-gas flowing through the turbine, that turbo is spinning. You can make a well-balanced turbo spin by merely blowing-through it.Doesn't a turbo, whether bike or car, only kick in at high rpm? And if so, isn't that the definition of bhp? I concede I may be completely wrong here but I've never heard of anybody adding a turbo to any engine to increase TORQUE. Wish I knew a good engineer...
As to when it "kicks in" - that's all relative to compressor and turbine vane size ("trim"), pitch, weight, the bearing assembly (ball-bearing turbos turn more-freely than plain-bearing, hence "spool" more-quickly) and the compressor and turbine housing inlet to outlet ratios. It's almost more guesswork than science choosing a turbo.
As to the torque question: the reason big-cube engines (usually) make so much torque is because, due to displacement - they're moving a lot more air (and consuming a lot more fuel) relative to a small-displacement engine. If you "stuff" more air (and fuel!) into a small-displacement engine via a turbo or other supercharger (or Nawz, Baby!), what do you suppose happens to the torque-output of that engine?