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We pull from the knee area towards the groin in order to place more pant material near the groin area. With more pant material near the groin area, there is less pant tension near the groin, with less pant tension near the groin there is more space for the groin, thus we are more comfortable. It is never comfortable to have material tightly stretched near our groin area because it is uncomfortable, and it may reveal our limp genitalia pressed against it. It is not an effort to hide an erection. For that there are other techniques.