During take-off there are combinations of height/airspeed that don't provide sufficient stored energy, or time, to recover if an engine should fail.
On a graph of height vs IAS the no-go area is a bulge on the vertical axis, starting at some smallish height/0 kts, expanding to the right & upwards then finally contracting to disappear as sufficient height is gained.
There is a a second bulge on the horizontal/IAS axis that starts at some knots then, expands and then contracts & disappears once past a certain critical speed.
The upshot is that there is a 'window' of heights vs. speeds in which it is safe to operate ie to take-off, accelerate & climb.
For landing, a vertical descent can cause the helicopter to descend through its own downwash. If you imagine how much loss of efficiency a fan or propellor would experience if it was moving backwards with the airstream it was trying to accelerate then that is similar to what the rotor blades would experience.
It can become severe enough that no 'new' air is being accelerated through the rotors. Instead, air that has just passed through the rotors curls up & around the rotor tips in a vortex & then feeds through the rotors again.
This results in a massive loss of lift producing ability for the rotor disk.
[ 11 July 2001: Message edited by: Tinstaafl ]