STAS is a 4 letter acronym for ...
A"ve A Peek
... and so is a sensible way to comport yourself on the modern battlefield.
Enemies of the British squaddie are unlikely to follow this simple rule of thumb. This is quite important should British military personnel be shot at in sandy or rocky places. When the Johnny Foreigner you just shot at and missed stands in the middle of the road thrusting his crotch at you in contempt ... you should take advantage of his lack of knowledge of STAS and slot the cnut properly.