Absolutely treat it the same as you would a bitted bridle.
I have found that most horses that will flex  nicley with their bitted bridle, pick up the flexing in the bitless automatically. I actually taught my mare to flex to the side in her rope halter first, (I was standing at her head, not mounted) and when I first tried it in the bitless bridle she hung on it at first, closed her eyes and acted like she was sleeping. I didn't tug, just waited, she finally gave me a little give, then I released. I kept asking for more, spent only five minutes each side. Next day it was automatic.
Then I did it under saddle a stand still, then walk, trot, and when she mastered that I tried it at a canter.
I never worked on it for over 10 minutes at a time, and she really retained each lesson. Now, she can feel when I am thinking about flexing her and repsonds swiftly, and very light.

I have used this move (One rein stop- stop your feet, do not spin)
in an  OH situation!!!!!!  I was at a mounted shooting event, My horse is used to gun fire, I use ear plugs on her as well. ( I didn't put them in yet, as we hadn't started) We happened to be shooting at a gun club that was holding thier own SASS event. there was gun shots to be heard but they ver far enough away my horse wasn't concerned. (we mosied into the ring to warm up, just trotting along, when all of a sudden just 200 feet away separated by a huge burm, (you couldn't see all the people, guns, targets etc. ) a barage of gunfire exploded, pow pow pow pow clang clang! pow! and this went on for what seemd like an eternity, but was probably two minutes. It was like there was a firing squad killing these targets my horse and I could not see.
Well she wanted to get out of Dodge , she bolted sideways, I immediately slid my hand down low, and she flexed, but her butt started spinning real fast. (It is not supposed to, I wasn't asking it) after about five spins, she stopped, she wanted to still leave, was all on fire, jigging. I flexed her again, she didn't spin this time, when I released she wanted to go again, so I flexed agian to a halt and  choose to dismount (at a stand still- the beauty of the one rein stop). I worked her on the ground a few minutes, then mounted up.
I really could in no way balme her for her reaction.
When the competition started, she went fine. I was  shooting my 45 caliber Ruger Vaquero's but evey time we came close to that burm she was leary. The final run, a wild turkey flew right in front of our path, not 12 feet from her face as we were running. We didn't even blink an eye!
Several people said "Did you even see that turkey that flew on front of you"? Becuase we didn't miss a beat. I said "Of course we saw it. After that gun fire, that was nothing!"