The button is called HYD TEST. In the FLight Manual we got for the course it is still called ACCU TEST.
You are right, it is the accumulator test circuit.
According to the instructor, some accidents happened, because less than well trained pilots "paniced" and pressed "whatever" HYD button when the hydraulics failed.
By switching the collective HYD switch off you get non-hydraulic control back, but not on the TR, as the compensator is depleted too - that would only apply if you have a TR-control loss and want the pedals/TR somewhere centered, you still don't get control back, but you have a chance to run it on.
Without hydraulics this is going to be a hard game. (...as was wit the NY newscopter - this one went around a few times, before "coming to a stop"...)
You are also adviced to land as soon as possible if you have a HYD failure, because you will fatigue in short order.
My instructor said 2 min without HYD every day will take care of any workout needs you have!!
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3top
PS: Watch the B3, this beast will destroy any piloting skills you might have had - there is just NEVER any lack of power - just pull collective. If you get just somewhere near the yellow arc you know you are severly overloaded
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Also the Koala is no good for tall pax, and have yet to figure out where 6 pax would put all their feet....