In the movie I remember, the protagonists use explosives, to which they tie a piece of string, such that they detonate when the string is pulled. This works correctly the first time the audience sees it. Later, they rig up explosives again, but the antagonists either arrive earlier than expected or are waiting in ambush, and sever the string before it is pulled. The antagonists begin to apprehend the protagonists, and one of the protagonists pulls the string, only to find it useless. He then follows the string and comes face-to-face with an armed enemy.

I vaguely recall this as a WWII film with German antagonists. In any case, the protagonists don’t have the strength to openly confront them; they are saboteurs or partisans, not regular troops. It’s possible the string was connected to some means of destruction other than explosives. It was definitely a physical string though, not an electrical cord or a gunpowder fuse.