How does a dog knot feel?

A dogs knot has nothing at all to do with discharging semen. A dog starts doing that as soon as it starts thrusting whether it has penetrated anything or not. Dogs don't cum like humans do. They begin squirting small streams of as they thrust and continue until the act is completed. The knot serves as a plug in the females vagina to hold the cum in for as long as possible. Female canines have eggs maturing for fertilization over several days time unlike a human who normally has only one egg at a time move down from the ovaries. A dog will continue breeding during her cycle as more eggs enter her cervix. That's why puppies can look so different. They can and frequently do have different fathers. Smart breeders know to allow their dogs to breed more than once over several days time to increase the number of puppies produced.