Well, the dog goes into this position mostly when it is petted while laying down. Your dog is basically submitting to you and is encouraging more petting. This position is a relaxing, submissive and pet me position. Your dog is signalling you to rub away, don’t stop that groovy hand massage all over its body. The odd thing about dog going on its back and opening its legs while paws bent is that it will do this also if you begin to pet it with your feet. Yes, I do pet my dogs with my feet when I see them laying down and wish to give a little doggy love or just being playful.
Once you have stopped petting your dog, it will simply roll over and stand up. It is like your dog have gone into a petting trance.