Taking each other

[B_subgirrl]

The upper 1/3 of the vagina opens up like a tent and is usually not rubbing the cockhead at all during sex if she is aroused. It's natures way of having a man want to pull his cock out a little bit to get stimulated by the lower 1/3 of the vagina so more vigorous thrusting will occur and ultimately make him ejaculate. I hope that makes sense.

Whilst much of what's on this site is crap, it has a great diagram of a tented vagina on it.

 

[OlderGuy]

Combined with penile stimulation in the anterior and posterior fornices (A-post and deep spot or cul-de-sac or backstop), which will cause a woman to lubricate deeply as opposed to up front from the Skene's glands, it can make it easier for a large penis to be accommodated without causing impact pain in a woman's cervix. As MR said, it allows for more vigorous sex later in her arousal cycle leading to climax and deposit of the semen at the woman's cervix.