My horse tends to be behind my leg - so my coach is always having me really push him into the contact to the point where I have a very strong feel in my hand - he is still loose and supple, however is really into the contact and driving from behind.
How one learns to have a good, supple and strong contact is a process... first of all you must learn to move the hands so that they are steady relative to the withers. You cannot follow the mouth. You want the forehand to move as one piece and so the hand must encourage the neck to move with the withers. How this is accomplished is with a bucking strap, holding onto the saddle pad or, if your arms are long enough, resting the knuckles firmly on the neck, directly infront of the withers. Always think about pressing
the hand forward and down, without moving your elbows from your hips or leaning forward.
Once you are stable in having the position of the arm correct, you must learn to correctly position and use the hand to create suppleness. The thumb is ontop at all times, and the hand is rotated longitudonally such that the pointer finger is closer to the front of the horse than the pinky finger. From here it is possible to create a great deal of flexion with the wrist. You can easily take up 2-3" of rein without moving your arm.
Now, the fist must be entirely closed around the rein. No open fingers that allow the rein to slide through!! Then you can use your fingers for aids - when they are so loose they are always inadvertently giving the horse an aid, making him dull to the bit. This is where your little bit of suppling and flexion comes from.
The position of the hand is very important and has many other implications which can be discussed in detail. However, when learning it is better to keep in mind only the simplest position and focus on making that as supple as possible while maintaining the arms' and hands' positions.
When you have the arms and hands set up correctly - it is good to think about riding "through" your hands with the seat - up to the contact. You must never think backwards with the hand - always remember to press it forward and down.
If you would like to watch someone whom I think has just the loveliest soft, but effective hands - watch Edward Gal. Good luck!!