It's no doubt referring to the "whap whap whap" sound of the tail rotor's wake causing interference patterns with the main rotor's wake. Next time you're near

It's no doubt referring to the "whap whap whap" sound of the tail rotor's wake causing interference patterns with the main rotor's wake. Next time you're near a helicopter in flight, notice that the "whap whap whap" chopping sound is more prominent at certain angles. Because it's constructive interference the pressure wave can be quite large and sharp... just like a gunshot, I guess?