1) Put your feet in an L, front foot pointed towards your training partner. Note that this is an L and not a T: your legs should not be crossed, and your rear leg should be able to kick straight forward past your front leg with no interference. Distribute your weight 50/50 to start.
2) Imagine the top of your head is suspended from the ceiling by a string, and everything hangs from that. Picture a life size model skeleton in a high school biology classroom. It hangs limply from the hook on the top of its skull.
3) Imagine a heavy weight is suspended on a string from the bottom of your pelvis. It's constantly pulling your center of gravity down against the string your head is hanging from.
4) Now imagine two strings attached to your wrists pull your wrists up and forward, towards your training partner. Your shoulders and elbows remain relaxed and hang loose.
As your legs work like hydraulic pistons to push differentially against the ground, changing the location of your center of gravity and turning your body left and right, you maintain these feelings of head suspended from a string, weight suspended from your pelvis, and wrists (not arms or shoulders!) held up by strings.
This should help you maintain the feeling of full feet and empty body that you want in contact flow. You feel like all your strength, vitality and energy has flushed down out of your upper body and into the bottoms of your feet, leaving your upper body like an empty plastic bag, weak and almost sickly feeling--while still suspended from a string on top of your skull.
Give it a try and report how it goes!