During the certification testing it must be demonstarted that the helicopter is controlable in 17 kts winds from all directions. Direct tail wind is not always the worst case. During this test the engine charcteristics requirements i.e. free from stalls and surges, must also be complied with. If reingestion is a problem it must be rectified, usually it takes slight tweaks to the shape of the engine cowling and or exhaust stack(s).
So operating at high power with quartering tail winds over 17 kts MAY result in high T4's or possible a stall in some helicopters.