As you roll from one side to the other you can feel a definite weight shift. I visualize it as laying on one shoulder, then releasing and shifting my weight to leaning on the other shoulder.
Just something I've personally found useful...
As an exercise you can have some bounce letting your body raise to the surface as you roll slowly and drop back onto the other side. If you feel you're landing on your shoulder, then you're "swimming downhill" - if you feel like your weight is dropping on the length of your arm then you're "pressing your buoy" OK, but not optimal, and if you fell your weight dropping on your hips then you probably don't have your balance right.
Note I'm not suggesting you should actually bounce in your regular stroke - it's just an exaggeration to get a feel for the weight transfer.