Hi Blakmax,
I presume you are referring to the G-RIDL accident.
bad running engine so the pilot elects to make a precautionary landing and descends (which is witnessed). To slow the aircraft down before the final landing the helicopter would have been flared which lowers the tail rotor - if this is done too late (ie too near the ground) then the tail rotor would hit the ground and the aircraft would lose yaw control. With a downslope the chance of hitting the tail rotor increases (for obvious reasons).
Hope that helps explain a bit for you.