Yes, we've got the spelling right. Trails, not Trials. It's a race to prove which hunt can field the fastest and most accurate hounds following a drag scent, and it's been a fixture of the New England foxhunting scene since 1923.
Each hunt may enter up to 2 couple of hounds. Competing hounds may be cheered on and handled by their staff at the starting line, but at no other point in the race. Patrol judges are stationed at strategic places to penalize hounds that skirt or cheat (i.e., take shortcuts off the true line to get closer to the front runner).