Pure speculation. When releasing the bolt, the bow may flex hard past the resting point, and maybe the second string limits that flex to avoid damage.

That makes sense. Was also wondering if it prevented the arms curling too far forward when unstrung, or as a foot rest to preserve the arms when bracing to draw back the string.

I should have guessed you'd be able to tell us. Not a clue here, but I can't wait to find out!

I don’t know! Genuinely curious.

I'm guessing it accelerated the bolt by exerting extra tension on the bow to snap back

If it were a musical instrument the answer would be sympathetic resonance.

I'm just used to you knowing weird stuff (a compliment) so I assumed you had it.