@poit57 said:

What did May break at the end of the opening scene of the finale that made Yo-Yo break into tears, collapse to the floor, and let go of the centipede device?

She broke the alien vial containing the alien serum that made them unstoppable and sent them into a Berserker rage, it was introduced in the first half of the season. They had two choices, use the Centipede serum to save Coulson or combine the Alien Serum with the Centipede serum to kill Talbot. That is why Yo-Yo was distraught when May destroyed the serum, she thought that was the only way to take down Coulson. In the end Coulson did not take the Centipede serum, he gave it to Daisy who used it to takeout Talbot.

Thanks. I just re watched the scene after knowing what I was looking for. On my first viewing, my focus was drawn toward the test tubes at the other end of the tray and I didn't notice the vial of the alien serum that we were supposed to be looking at with May.