Englishal.
Because each of us have differing personal lines in the sand on what is realistic and what is safe, it is difficult to say what technique is right and what is wrong.
I noticed with regret that hardly had I originated this subject than someone starts sniping and accusing me of "selective" paragraphing. Obviously Pprune pages are not the place to repeat word for word whole chapters from technical manuals on engine failure techniques - hence my obviously inadequate attempt to reduce things to essential meanings.
So please chaps - accept this subject as a thoroughly absorbing one for flying instructors to discuss, and try not to let it degenerate into carping and heavy criticism. This latter point is not aimed at you Englishal - but those who have taken the time to add their bit.
Re pulling the mixture on a twin during the takeoff run in order to test competence at an abort. There is significant danger of mishandling if this is done - which is why it was rarely if ever done in the early jets before simulators came into being.
Unless the student reacts instantly to snap both throttles back then the aircraft can suffer severe side loads on the gear as the aircraft veers to one side at high speed (say 10 knots before lift off speed). The slightest hesitation in getting both throttles back will require swift intervention by the instructor with brakes and rudder.
If it is accepted that some students are slow to react correctly to a simulated engine failure after takeoff - whether a mixture cut - or a throttle closure - then some students may have the same problem when it comes to pulling an engine in the middle of the take off run. No point in testing a students reflexes in that manner if less than one second after failing the engine on the runway the instructor has to take control less the gear is wiped off. There is no point in practicing bleeding. Someone might get hurt! I leave the risky stuff to the bold instructors. Better still leave it to the simulator.