underlever

noun2

1. •
   A lever behind the trigger guard of a rifle

   • Incidentally, the current underlevers developed and produced after World War II are vastly improved over the prewar types, though millions of the latter are still used. — 1957
2. •
   A particular piano component.

   • The wippen, a mechanism of underlevers, controls the jacks that push the hammers up to the strings. — 1996