DCHP-1 Online
key-piece 
n. the log which when released will free a jam (def. 2).
See also: jam ((n.)) (def. 2) key-log (def. 1)
- 1891 The "keypiece" of the jam was fully exposed, and, once it was cut in two, would no longer hold the accumulation of logs together.
- 1964 Many a man risked his life in breaking up these [lumber] blockades with the handspike by skilfully extracting the key-piece of the jam. . . .
- 1891 The "keypiece" of the jam was fully exposed, and, once it was cut in two, would no longer hold the accumulation of logs together.