A hammer strut is a part in the firing mechanism that connects the trigger and hammer and moves the hammer into the firing position when the trigger is pulled.

or

A hammer strut is a part that connects the hammer and the mainspring and as such transfers energy from the spring to the hammer when needed.

Footnotes / Sources:

“Glossary ⇒.” Sporting Arms and Ammunition Manufacturers’ Institute. Ed. SAAMI’s Technical Committee. Web. 15-18 Jan. 2011.