This article is from the Lure Coursing FAQ, by Bonnie Dalzell with numerous contributions by others.
A slip lead is a specialized collar and leash combination that allows
the handler to rapidly release an excited hound without breaking a
finger or damaging the hound. The best leads have a broad collar that
is several inches smaller than the hound's neck with relatively heavy
3 inch brazed metal rings on each end of the collar. The leash portion
is strung through the metal rings in such as way as to hold the hound
until one end of the leash is released. Then the hound is free.