I think it's in relation to impossible shirt, which I think is supposed to indicate a shirt with "breast cavities". Similarly these panties magically indent to stick to the skin attaching the scrotum to the penile shaft, so I guess that's the idea.
Underwear doesn't seem so impossible. Her thigh could be pushing the right testicle upwards, and the penis could be pushing that lower, possibly more slack, part of the underwear down, while causing tension at the top (like slid-down socks meets non-babydoll T-shirt on a busty thin girl... or somethin').
