In a snake swim, you swim 2 lengths in each lane of the pool. For the first length, you stay on the wall half of the lane and on the second length, you swim on the other side of the lane. After you swim two lengths in one lane, you duck under the lane rope and start swimming in the next lane. Continue until you're on the other side of the pool in the last lane. So in a 6-lane, 25-yard pool, a snake swim would be 300 yards. In an 8-lane, 50m-pool, it could be up to 800m.

The key to having a good snake swim as Tyson pointed out, is to have everybody accurately seed themselves. I've had very good luck with this and have had no traffic problems in the 2 snake swims I have done.