There are 2 things that you must have in mind during approaches with helicopters. 1. There is an area in airspeed (about 16 to 25 kts), where transitional lift phenomenon takes place. At and below that speeds, and at an altitude greater than 50 ft, the rotor goes into its own downwash, so there is a "sink" to the ground. 2. At low speeds (<40 kts) and hi rate of descend (more than 500ft/min), also the rotors goes into its own downwash. So, the best technique is to maintain 40 kts till about 10-20 ft above ground, and descend with less than 500 ft/min.