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This position is a fairly stable one and has a couple of variants. As you may notice, in the first three images, the shooters are sitting with their legs open and in the third picture, the shooter has his legs crossed.
While this is not as stable as the prone position, this method has a couple of advantages over it. Unlike the prone position, this position is faster to get into from a walking position. It also has the advantage that it allows the user to see over low brush and tall grass, which is sometimes a problem from the prone position.