its warm and cozy, and is a great way to get her fired up. she can just feel you resting there. sometimes she'll take her legs and squeeze em together as if to apply more pressure to herself, or if you tense up your muscles a bit and lift your dick, it presses more firmly against her vagina and is enough to casually stimulate her, almost as if to tap her on the shoulder and request that things go further.
works like a charm. relax, pop a few dick lifts, and she's all "oh fuck yeah we're doing it right now".