Dog style
From BeastWiki
Dog style refers to a sex position often seen in animals especially dogs. Dogs usually assume this position when they start to mate. In this position, the receiving partner bends down on all fours with slightly arched hips to accommodate the penis of the penetrating partner. The dog will lung onto the back of the partner, and begin to hump, slowly at first, than faster as the penis enters the vagina or anus. Once he has penetrated, the dog will be "tied" to the partner, then lift one of his legs over the partner and face away from the female (this usually only happens with dogs only).
This is done the same way in bestiality. A receiving participant bends over on all fours and accommodates the penetrating participant from behind. Actual penetration may be vaginal or anal. Once the dog penis inside the vagina or anus, it will hump until it ejaculates.
There is a reason why both female dogs and humans would assume this position. A male dog's antinomy has an organ at the base of its penis, near the shaft (the place where the penis goes when it isn't out) called the bulbus glandis. This organ will swell with blood, causing it to lock it's self inside the female partner. They do this because a dog doesn't just ejaculate for a few seconds; it usually takes a couple of minutes. They don't release all of their sperm all at once, so they must remain inside the female until they are finished ejaculating. One of the benefits of the doggy style over the mercenary position is that if you allow the "knot" inside you, their is a higher chance that the male or female receiver will reach orgasm.
The size of this organ varies among breeds, usually coralation wiht the size of the partner. So the larger the breed, the larger the knot.
This position may also be called rear entry, doggie style or doggy style.
