Pretty sure I've done it before. Its a compression/expansion/compression exercise. The two bounces (trotting in) get you bouncy, then you push across the the hogs back and compress again on the out of the two bounces. You build up to it in a very progressive manner---you do not just go right through the line! The low wide oxers at the other end of the ring is the opposite....the wide oxers get you expanding and then the bounce in the middle get you compressing...so expand/compress/expand.
He will adjust the distances some depending on the horses in the group and he can change it up a ton of ways (taking out part of the line).
** Tact is the ability to tell someone to go to hell in such a way that they look forward to the trip. ~Winston Churchill? **