My friend is doing a 5yr old class today at The College and I believe that it is a little like a "showing" thing... for want of a better explanation....you do walk,trot canter on both reins altogether and they assess the horses way of going, ie. expression in paces, balance and rhythm, presence etc...
I was stwearding the collecting ring at Somerford Premier League the days these were on and it *seemed* a bit like a showing class. Horses went in 3 at a time and the judge would stand in middle and ask you to do things and would focus on a particular horse each time. It didn't last any more than 10 mins. The judge at Somerford then called top 10 back in where they trotted round and would pull the horses into line in order.