I’m going to quibble and / or question something someone said above, about resisting until the horse yields, yielding and then repeating.
This is 100% how I was taught 20 years ago, but in less experienced, less competent riders (ie people like me) it can lead to an overuse of the hands, hard hands, a horse that leans and wants the rider to carry him, or a tug of war between horse and rider.
Since getting back in the saddle again three years ago I have been taught that today’s lighter method is the momentary resistance we have described, followed immediately by a release, whether the horse yielded or not. The resistance should be only for a stride ideally; more than that is holding. If the horse doesn’t “yield” or give the response you wanted, ask again but with stronger, sharper aids this time. If that works, go back to the lightest aids for the next half halt.
Anyone else learn or use that method?