With regard to practice turnbacks (or indeed any simulated emergency), I remain unconvinced of the value of the exercise if the handling pilot instigates the situation. Be it an engine failure in a single engine airplane, chopping one throttle in a multi or simply pulling the cable release in a sailplane on a winch launch or aerotow, if the handling pilot closes the throttle/pulls the release etc then their mind is already in front of the situation. This is not the case in the real thing, which is why the instructor should always instigate the simulated emergency.