Rotor blades can flex sufficiently to endanger anyone under them when RRPM is less than flying rpm. There is, and unfortuately I do not have a copy, a photograph of a Wessex rotors running on a carrier with the forward rotor blade flexing to about 4 feet of the deck. Blade strikes of tailbooms are not uncommon during starting and particulary stopping rotors during windy/gusty conditions. Hiding behind a building to be out of the wind is not the best place to close down as eddys and gusts can be more destructive in these places - stay out in the open if possible where the wind is more constant.