MSI GeForce GTX 680 Lightning review - Product Showcase

Very visible is the blue round GPU reactor. The GPU reactor functions as a power supply module which, when installed on the back of the GPU, it increases current volume by 88%, reduces power supply noise by 13%, and should improve overall overclocking stability. It is actually an add-on board which you may remove. Let's do just that.

You can remove the plastic cap and then see a small PCB. That's actually removable as well:

The PCb itself hosts a battery of tantalum capacitors that are used to filter and stabilize GPU voltage much like the NEC/Tokin Proadlizer film capacitors we all know. Be careful when you plug it back in. 

Once powered on the Reactor module also features a few blue LEDs onboard, so when it is installed with the GPU Reactor cap in place, it has a nice blue glow emanating from it. The GPU Reactor can be removed completely if needed or preferred.