The vulnerable position is when someone is bending down tying their shoe. NO, YOU DON'T PUT YOUR PENIS IN THEIR BUTT, instead, you push them over. This is such a vulnerable position because the person's weight is usually on one knee, making it difficult to balance, and their hands are being occupied with tying their shoe, thus when you push them, they won't be able to catch themselves in time and they fall over.
This was made famous when during Liam's freshman year when he was at racquetball practice. He was playing against a junior named Justin, and Justin bent over to tie his shoe. Liam looked back at Adam through the glass and made a pushing motion. Adam gave him the thumbs up, and Liam ran over to Justin and pushed him over, much to the chagrin of everyone who was watching.