It's a critical part of the blowout preventer (BOP) and it could have made all the difference, had it worked. It's called a blind shear ram and it's supposed to stop the flow of oil if all else fails. Huge pinchers with blades close around the drill pipe and cut it off. But when Deepwater Horizon's crew tried to engage the pinchers they wouldn't close all the way. Over and over the crew struggled to make it work, but in the end this last line of defense failed.
Once the blind shear ram failed, the blowout preventer simply couldn't do its job.
A viewer asked via Twitter if the Atlantis oil rig is using the same blowout preventer as the Deepwater Horizon."
According to BP, there is no blowout preventer at play with "Atlantis" because blowout preventers are only used during drilling, and the Atlantis is actually in production now. A different system controls the flow of oil. It's a series of valves in the well and on the wellhead called a "Christmas tree" because it looks like a busy Christmas tree.