This is another one of those foundation skills that could profoundly influence your training success. Ok, I say that a lot, which is why these skills are on the foundation list in the first place, but I really mean this.
Not having this skill (or to using it) is also the number one reason why dogs rehearse and learn bad behaviors in class, practice, on the agility field and as I commonly see, at seminars.
Teach your dog to put his collar in your hand or at a minimum to happily allow you to reach for and hold them by the collar. Simple as that.
If you combine Hand-in-collar with your invitation to play voice (ready?) - it is the start of a great game and dog should be straining to get to what you are holding them back from. This is enormously helpful for building speed and fun into the behavior you are training. By the way, In this scenario, do not tell a dog to wait and then use restraint – they are opposite mindsets!
You should also teach your dog that your hand in their collar, when you are calm and not inviting them to play is a marker of a time to be calm. It is also a happy event, not a correction. This is enormously helpful when you are done rewarding your dog after an exercise and need to get your thoughts together as to what to do next. Or when your instructor is blabbing at you so your dog doesn't get good at ignoring you and wandering off on an agility field. Or when you have made a handling or training mistake and need to regroup. Or when you are distracted. In short, any time when you are disconnected mentally from your dog, use your hand in their collar to maintain the connection so they don't get good at the feeling of being disconnected, wandering, visiting, peeing, sniffing, hunting for treats etc..
Your dog will know whether to be calm or excited by the difference by the energy that you bring to the game.