I assume it's not just our spaniels which can exert 'dog gravity'? It's like some sort of additional power that sticks them to the ground whenever they don't want to do something or, more accurately, have something done to them, such as a bath or grooming/clipping. They can either fix each limb to the floor totally independently or they suddenly weigh ten times as much.
I believe it’s down to magic rather than power Willy? as labracadabradors’ are excellent at this phenomenon.
Is that something along the lines 'a dog is not just for Christmas'? You should get Boxing Day out of one as well.