This is a good thread Spyder. So many of the concepts that we deem as simple and obvious really are not when you are just starting out in dressage.
Suppleness, like rhythm, relaxation, straightness, throughness etc etc all comes from forward, and thus engagement.
You cannot achieve a horse that is truely on the aids, collected, and able to perform anything more advanced than a simple walk-trot transition unless you have engagement. Simple as that.
You cannot truely 'steer' a horse without engagement. Trying to give direction to a horse on the forehand is like trying to steer a road train around a tight bend. When engaged, the horses hind legs act as a 'rudder' (I am fighting to put this into simple, bite sized pieces!!) through turns. They push the horse around the bend and keep it upright. In a way, the forehand follows to hind end, rather than the front legs pulling the horse along, the hind legs PUSH the horse along. Much like a 4WD car compared to a front wheel drive car. The 4WD gives a far greater degree of control over the vehicle as the back wheels are pushing the car and giving direction, where as putting a front wheel drive on a muddy, slippery track will result in the car sliding out in the back end.