My horse, Hat Tricks, Anglo-Arab gelding, when I used my legs to ask for contact, reached forward gently until I felt contact with the reins just like he did for a snaffle though the contact itself was a little lighter. I followed all his head motions at the walk, and I did the three speeds of the walk exercise on contact, with him reaching out for a slightly stronger contact as he extended his walk, and with him shortening his neck slightly when we slowed down. I kept my fingers relaxed as I followed his mouth or used rein aids.
When I did the sitting trot, bareback, with the Weymouth curb, he raised his head higher than he did when I kept contact with the snaffle bit when sitting the trot bareback. To me, since I did not have a riding teacher at that time to guide me, that was a sure sign that I WAS NOT STEADY ENOUGH with my hands for Hat Tricks to give me the type of contact I would have with a snaffle. Hat Tricks was a good teacher, if I did stuff right he promptly did what I asked, if I did it wrong he would gently protect himself and tell me to do better. Occasionally he would decide that I needed a sharp lesson, and he would give it to me (wham on the ground.)
I did not canter with the curb on contact because of the limitations I set on myself since I did not have a riding teacher. If I was not good enough to do it at the sitting trot I was obviously, to me at least, not good enough to do it at the canter. So at both the sitting trot and canter the reins were sagging down maybe an inch, not so much that they were swinging too much and irritating Hat Tricks. All rein aids were gentle and Hat Tricks usually obeyed them before the slack was out of the reins. Since Hat Tricks was quite willing drop contact when my hands were irritating him or sling his head around violently if my hands were really bad, and I took his quiet head and steady contact with the curb at the walk as a good sign. We frequently cantered (sagging reins) and galloped often (sagging reins), bareback across the pasture. Hat Tricks did not sling his head around when I asked him to slow down from any speed, even a flat out gallop (1/2 TB, he could run some!)
At no time during my rides that summer did Hat Tricks lose impulse, he promptly obeyed my legs when I just flexed my calf muscles, so I did not need spurs at all. He was in front of my leg and quite willing to go faster sanely and obey me no matter what his speed.
Years later when I tried to keep contact with a curb with a 7" shank (a Walking Horse curb bit, a Tom Bass curb bit and a Show Pelham), the horses immediately went behind the vertical and I immediately stopped trying to keep contact with the curb and went to loose rein riding for the curb. I rode my Paso Fino mare for years with the 7" shank Walking Horse curb. I could not keep contact with this curb without her immediately going behind the vertical, but we communicated fine with sagging reins, gentle movement of the bit, and leg aids. When I neck rein with reins in both hands I look where I want to go, lay the rein against the horse’s neck, and reinforce the rein with the appropriate leg at the appropriate time. My little Paso Fino mare was a firecracker, always ready to go, Go, GO, but she would keep her speed down with light rein aids, even when she was looking for excuses to rocket off, keeping her head in front of vertical all the while.
My MS was already affecting me that summer I rode bareback with the Weymouth curb, not badly but enough so I could not quite ride up to the level I wanted to. My balance was already deteriorating though I still had enough balance to ride bareback, but my front to back balance was not ideal.
Because of this summer of riding Hat Tricks just with the Weymouth curb, my riding teacher has always been willing for me to use a double bridle or a Pelham even though my MS is MUCH worse than it was back in that summer. In fact she is willing for me to try the curb first with just my 7" shank Tom Bass curb just to see if the horse will accept a curb bit (both hands, sagging reins). I no longer can ride bareback since my balance is HORRIBLE now, but I still know how to ride in a curb without the horse going behind the vertical or getting upset. I do not ride dressage, but I do know it is possible to keep contact with a 5" shank curb, using both hands, without the horse going behind the vertical or losing impulse. I did it occasionally at a walk with the last two horses I rode with double bridles during the past ten years, and if I got to ride a horse suitably conformed for riding in a curb I would do it again, with the permission of my riding teachers.
If you’ve never been in a situation like that, count yourself blessed.