What is the meaning of Bucking_bar?

A heavy bar held against the bucktail of a solid rivet, which has been heated if necessary to soften it, and passed through the items to be held together, while a rivet gun hammers on the rivet's factory head; the impulses from the hammer, reflected by the bucking bar, then upset the bucktail to form a field head which is typically pancake-shaped.

Source: wiktionary.org