I'd say 'in synch' is retaining the relationships of the hands/triangle/chest through the ball. That does NOT mean it is a 'stiff' motion (hence the negative side of the word 'connection'). It is the timing and tempo involved in 'allowing' that relationship to stay in place.
If you stand with your upper arms to elbows against your sides, holding a club in with hands at chest high, clubface at eye level, there is a point from your chest to where your wrists meet. Being in synch is keeping a string from those points 'straight', and of keeping the angles formed basically the same through the shot. This isn't something you DO it is something that HAPPENS when your timing is on track.
One of the better ways to feel this is by swinging a broom, held down near the broom, with the handle against the left side.
To keep the relationships and angles in place, to keep that string straight, you need to turn everything back and through - together - in a flowing motion.
Take the broom, and put the handle against the chest, and swing back and through keeping it on your chest.
That is being 'in synch'
the trick is to make that happen in tempo, in 'flow' with the swinging club
in balance
As cdog says, get too far ahead or behind with the hands or body, and you have trouble.
Nothing about that relationships says the lower body can't be open, or even the upper body, as long as that string is 'straight', that broom handle stays in place.