During WW1 it became obvious that troops need a relatively compact mean to cut the barbed wire obstacles. Sapper tools were not issued widely and overcoming barbed wires was a disastrous part of any offensive causing numerous casualties. So Russia developed a special wire cutting device which was attached to the “experimental bayonet” and made it possible for any soldier to cut the wire obstacles.

During its testing, it proved to be pretty effective. Soldiers could cut the wires not exposing themselves to enemy fire. A soldier had to catch the wire between the two blades and push the rifle forward thus forcing the diamond shaped mechanism to work the wire cutter and cut the wire. The device had a spring loaded mechanism, which allowed it to retract back once the pressure was no longer applied.

https://www.thefirearmblog.com/blog/2016/12/27/7-rare-mosin-nagant-rifle-modifications-accessories/