It is important to locate and understand the height velocity diagram for the helicopter you will be training in. The "shaded area" is the area that test pilots have determined to be dangerous. The average pilot operating in the shaded area, may not be able to effect a successful autorotation in the event of an engine failure.
The diagram gives safe altitudes for given speeds. A pilot has three forms of stored energy (altitude, speed, and RPM). As you look at the upper shaded area, you will notice that giving up speed will force you to have more altitude to stay safe. Likewise if you give up altitude, you will need more speed to stay safe.
The lower shaded area forces you to have altitude after a said speed is achieved, to avoid high speed operations close to the ground. Operating at high speeds close to the ground, may not allow enough room for an autorotation flare.
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