In taiji, it is important to use intention, and thus mental images are very important. The mental image linking your hands to your feet is important, since the feet is the contact with the ground from which you derive your force. Mentally linking your hands with your feet allows you to bring the force generated by your legs into actual use (at the hands). When you come into contact with your opponent's force, mentally channeling it down to your feet allows you to neutralise it and change its direction. When neutralising, it is like making his force travel from your hands (assuming that is the point of contact) through your arms, down the torso and both legs to arrive at the feet. And then you make that same force travel from the feet up your legs and torso, through the arms to arrive back at the hands and in the direction that you want to bring your opponent towards.
So right now, the current focus of my taiji routine practices is to maintain the mental image of my hands being linked to my feet. Let's see where this leads...