It’s all to do with the accumulators . You don’t want to have hyd assistance in one plane without it in the other therefore you have to have them all topped up before you re set to Hyd on ... that works both ways so you cut Hyd off before the are depleted when doing a training exercise . Others can explain better then me but that’s basically how it is . The accumulators give you about three stirs of the cyclic giving you time to come back to around 60-70 knots then they will run out ( but not necessarily at exactly the same time )