How to handle HYD FAILURE  

Let's use the Bell 206 system for an example. Most hydraulic failures are the result of a leak. The system is high pressure (400psi?) and low volume (Less than a quartish?) so it doesn't take long for the system to fail after a leak. When you turn the switch off you are re-routing the hydraulic fluid, really a mist now, from the actuators back to the reservoir. That is a much smaller volume of space for the fluid to occupy and the pump is still being driven by the transmission. So, it is possible that a significant amount of fluid could become pressurized from the pump to the reservoir. If you were to turn the Hydraulic switch back on that fluid will go to one or more of the actuators applying boost in a short and probably asymmetrical manor. That could cause a loss of control.

This is not a Land ASAP emergency as the flight controls are working the same, they just require more force to move. You could conceivably fly until you fan out of gas. I would error on the side of caution though and land promptly because that pump could be running dry and getting hot with a lot of flammable fluid sprayed who knows where and plenty of wind to fan the flames if they do ignite. The pump's drive shaft is necked down between the transmission and the pump so it should break there if the pump seizes.

The AS-350 was harder to fly boost off because it is heavier and has more blades. After some accidents after extended boost off flight AEC changed their EP to disallow that. I hired a guy once that had failed out of Temsco's training because he couldn't fly for the extended period they thought was necessary for their Glacier tours. I thought "I can teach this guy!". Wrong, it was beyond some smaller pilot's physical ability.

The heaviest single system I ever flew was the H-55. When my instructor first turned the boost off I was seriously wondering how I was going to get that fat b*stard on the ground! But, slowing down, setting a landing attitude, and rolling on above ETL proved to be no big deal.