Don't know if this is related, but I find it easier to get any horse (bar a couple with very bouncy trots!) working in an outline in sitting trot! I always thought it was to do with better contact with seat (so you can feel more what the horse is doing), easier to keep a steady contact, easier to use your legs effectively...
Don't take it from me though - I'm no expert! Would be interested to hear what more experienced people have to add...