Spread legs that form an M shape.
The feet should be around ass level, knees around chest level, and legs spread to the sides such that if you traced them, you would get something close to the letter M. If you can only see one bent leg and the other is not bent, or if you can only see one leg whatsoever, do not use this tag. Basically, you want to be able to see the full M shape.
Be careful when tagging squatting with this tag, since it's actually difficult for someone to make the perfect M shape while trying to balance their waist/buttocks in a low squatting position. The upper legs need to be distinctly raised upward and the lower legs bent in such a way that you can make out the M shape, like in post #3775778.
Do not use for leg lock or wariza. It is also recommended to avoid adding this to two-footed footjob posts since you don't want the feet to touch each other, although there may be odd instances where the feet are not touching (post #3670342). A more appropriate tag would be two-footed footjob.