Well, millions of things! But here's a wee example from today at my yard.
I was doing stuff with Benson in the yard, and Elvis decided he wanted to join in. Clicker training doesn't work too well with 2 horses in on the action, one of whom is decidedly not doing what you want, so I asked Iain, who is quite new to horses, to hold on to Elvis while I worked with Benson.
So, Iain puts Elvis's head collar on and takes him to stand out of the way. I'm keeping half an eye on them, and am aware that Iain is managing fine, but there is some discussion going on between the two of them...
10 minutes later I'm finished with Benson, and Iain asks me the best way to respond to Elvis when he tries to nip. Iain is a competent dog trainer, interested in infinite levels of detail about horse behaviour and training, so off we go!
Firstly, the basic level - Elvis threatens to nip Iain, and Iain's response to this will either encourage or discourage his behaviour. So we talked about that a bit. If we are competent enough, we can certainly stop the nipping by ensuring that Elvis gets no enjoyment from trying this.
Then we stepped back a bit - why is he nipping in the first place? I watched the next incident closely. Elvis started looking over the field at some cows; within seconds his attention was totally on them, while Iain was standing passively next to him. Then Iain pulled a little on the lead rope, to get Elvis's attention. Elvis responded with a nip threat - the request for his attention was a bit abrupt and 'big' and his response was basically to say 'back off, that was rude and I'm busy' to Iain. So we talked a bit about keeping or getting back Elvis's attention, and how to do this.
However, stepping back again, I noticed that Iain's attention had been slightly diverted from Elvis before Elvis stopped paying attention to Iain!
So, in this case, Iain 'disconnected' from Elvis, so Elvis started paying attention to something else, then when Iain asked for his attention back it was irritating to Elvis and he said so. Once we got that cleared up, and Iain stayed focussed on Elvis, in this instance we had no more nipping.