The Wessex (S-58) engines are normally started with the rotorbrake on.
With both engines at idle, the brake is slowly released to a count of 1-2-3.
I was an engineer on an external intercom longlead when, the SAR pilot on a solo training flight proceeded the release the brake as quickly as possible.
The dear old Wessex (being a bit top heavy) proceeded to oscillate wildly, leaping from one main u/c wheel to the other, threatening to roll over.
I ran like hell, nearly strangling myself with the longlead in the process.
The a/c finally settled down as the rotor rpm cranked up & stabalised.
I returned & plugged into the intercom.
"WTF ?" I asked.
"I always wondered why they told you to release the rotorbrake slowly" he replied.