This technique gives a really powerful rowing stroke 

It’s more like. They should start to try to open up at 3/4 slide but if they are pushing adequately with the legs they won’t actually begin to open up at that point but more power will be generated.

@@DecentRowing OK thanks for the clarification much appreciated.

Very easy: hold your arms stretched until you have to pull, not to fall backwards. (Keeping up the blade pressure) Never pull your arms against the power of your legs.