I have had instructors who take the reins (at the bit) and help me with feel. I have also had an unmounted clinician lesson whereby she'll hang the bridle from her head and hold the bit in her hand and close her eyes (facing away from me) and I have to give instructions via the reins; then we swap places and she tries to replicate how I was holding the reins so I can feel what I was doing. I will say that I historically had ridden with a loopy rein - from my hunter days, I suppose - and would "throw away" the contact when we were cruising along in neutral. But that doesn't keep a constant communication with the bit, and it is actually quite surprising (in my hand when I was holding the bit) to have no contact and then some contact and then no contact and then some contact; as the horse, I wasn't ready for the next signal because the rider had to pick up contact before giving me an instruction and it was very confusing. That was an eye-opening lesson for me.
Yes, absolutely I use one rein at a time, or both, or one more than the other, or sometimes strongly to get the point across, or whatever my horse needs at the moment. I guess I don't have a "one technique fits all" approach. I'll do what my horse needs so I can get my message to him . . . I find it is a constant conversation that is only "silent" when we're on a break and moseying around the arena.
As a yoga teacher for riders, I've also done lessons in my class to work on feel and soft, moving elbows. Use a lead rope or yoga strap or bathrobe belt/tie . . . something long enough that you can wrap it around your feet and have the ends be "reins." Sit on the floor with your legs straight out in front of you and your body upright (you might need to put a pillow under your bum if this strains your back). Wrap the middle of the strap around the balls of your feet and take the ends of the strap in each hand like reins. Sitting up straight, move your feet in a pointing-and-flexing manner (but you might not point all the way because you don't want the strap to fall off) - just enough to create movement that you'll need to follow with your hands. Close your eyes and notice what you feel in your body. What does your usual contact feel like to you? How light can you go with your contact and not have the strap fall off? Where do you notice any blockages - shoulders? Elbows? Wrists? Are your hands relaxed? Are you breathing? There are all sorts of things to be aware of in this simple exercise.
I strive to be a better rider with my balance and position - sometimes I really have great moments where I feel at one with my horse and we are totally in synch and I feel like "that's IT!" It is those moments that I keep trying for, working on myself and what I can do to get there.