Who needs hands?
Never, never snatch the controls off any student at any time!
If at the hold off stage you decide that you must take control then this should have been agreed with the student before the approach was commenced. i.e. "I will allow you to continue the approach and the landing for as long as possible - should it be necessary for me to take control at a any moment I will do so and say I have control, but if I should take control in this manner please understand that I have left it as late as possible for you to correct any error yourself". However, doing this should always be a last resort but the student will now be aware from the onset of the possibilty and therefore maintain confidence.
Restricting the controls or nudging can only confuse. The student must always know that the feel and response to control inputs is theirs and theirs alone, if they are ever to learn. With regard to the approach and landing the student should be encouraged to identify errors and make their own decision to go around. Continously allowing the student to continue beyond their ability is not good and unwittingly will encourage a blinkered attitude such as continueing to a landing at any cost.