So, you're asking for a cue for collection, right? Well, for me it's hard to bring it down to one thing. I have cues for collection - body language most of them, but also voice cues, touch cues, ... But when I think about it, I think the most important cues for me are of a situational and emotional kind.
Collection for me is not just a certain way for carrying oneself - it's not just happening in the body. It's a state of body and mind. A state of collected energy, so as to use the body most efficiently, but also to use all the senses to full effect. It has a lot to do with awareness, balance and poise. In a lot of yoga exercises, I get a feeling what it must be like for a horse to be collected and I understand why they love to learn about it.
So where do I want to go with my argument? I think to seek collection only via shaping of the body and the movement falls utterly short and misses half of what collection is about in my opinion.
So what I do is to set the stage for collection first. Whether I want a playful type of collection, a competitive one, or maybe even a defiant one, I use play, wild play, teasing, provoking. Also certain movements help to set the mood, like rearing, school halt, Spanish walk and so on.
It's in those situations where I exercise collection and there is no specific cue that set it off, but the whole situation. Over time, those situations became more frequent and collection and energetic movement became more and more the usual mode of interaction.
What I try of course in those moments is to be collected myself. Maybe that becomes the real cue in the end?
Why do I go to these lengths, instead of establishing a simple cue like a touch of the croupe or similar? Because I believe that that's the only way to "exercise" real, natural collection. It's the only way to ensure, that it's done by the horse himself and not just instilled by me.
Both our horses started to lift their backs and tried to collect from the first moments of riding, without any cue from us.
I believe that horses like to collect when being ridden, because they know it's the best way of carrying a heavy load. But they need to learn it on their terms, in their time and have the chance of feeling the difference in a stress-free, playful situation
Now, that wasn't the straight to the point answer you'd probably wanted, but I think the most things in horse training are best reached by a devious route