I always work with my pups when they are very small and all of them will allow you to lay them on their backs when they go to their forever homes. Some people don't have the time to do this I guess. I start by holding them and kissing on them while on their backs and they LOVE IT
I can hold my 5 month old on his back in my two hands out away from my body and he is fine because he trusts me. Sorry if this is too much info