And additionally there are the machines with yaw compensators -
The scenario - loss of hydraulics - pump belt has broken or pump puked.
People go - turn off hydraulics - inadvertently or ignorantly hit the HYD TEST.
Due to the requirement to unload the compensator after shutdown - pushing the switch will unload the compensator - which you do not need in flight and
HYD OFF will NOT unload the compensator. HYD OFF or HYD failed AND HYD TEST will dump the TR compensator hence the instructions above.
Reset HYD TEST then HYD OFF to recharge the compensator when training.
The reason for the compensator is that as they needed more thrust from the TR with the increase in gross weight they added a tab to the trailing edge.
Uncompensated machines have the same TR but at the lower gross weight it is manageable.
Heavier machines - you will lift yourself out of the seat before you get enough pedal fighting against the tab.
Back to the issue of the now dumped compensator - you ain't getting it back because you have no HYD pump.
More than a few have ended up on their side because of this and some very high time guys that I know personally.
DUAL HYD machines also have the ability to get airborne without the TR compensator charged and no pressure as a few have found that are no longer with us.
The preflight test has moved to post flight to try and address the issue on DUAL machines.
You need a very clear understanding of the issues on this type.