From a circle of four (retain hand holds throughout): A designated couple will duck all the way through the arch made by the other couple and immediately raise inside joined hands, turn back to go under their own arch and finish facing the opposite couple with hands lowered about to waist level and crossed in front of them and all are circling left. Dancers can "Unroll" the barrel by going through the reverse of the previous actions--if they're lucky. The call for that is Roll The Other Couple Through.
This definition is from The Handbook of Modern Square Dancing by Jay King.
It is provided here for informational and educational purposes only.