my response was like Allison's. I thought , "what?" seems backward to me.
I dont'think you should ever think of it as "tension". But you need to have "tone" in your wrists. I also think that there is tone in your forearm, and shoulders. Your elbow needs to be very moveable.
Tone means that the muscle is not flacid. You might think of it as roughly halfway between being totally flacid and totally rigid. Or mayve a bit less than halfway. But even a relaxed part of your body , such as your buttock muscles, should not be flacid.
If your wrist is really loose you will not have enough tone in it to be able to feel the horse's mouth and how the bit is in his mouth. THIS is necessary for you to be able to follow the mouth. IF you don't have a firm contact with the mouth, you simply cannot follow it. If you cannot follow it, you will alternate between tight/ gapping contact . . ..tight/gapping . . Tight/gapping, and each time it hits tight, it hurt the horse because HE cannot follow your hand, either.