Loader Valve REGEN Feature Explained

Regen is a feature of most modern FEL (Front End Loader) valves. It is on the bucket dump (joystick far right) circuit and is also referred to as "Fast Dump". The reason it is nice to have is that, without regen, the weight of a filled bucket can actually "pull" the bucket down faster than the fluid can enter the other side of the cylinder. This will create cavitation (sort of an air pocket) and give the bucket a "floppy" feeling until the joystick is held in the dump mode for a few seconds to refill the cylinder. 

Regen solves this problem by actually filling both sides of the cylinder at the same time with hydraulic fluid. But how will that work you might ask? Well, because there is more piston surface area on the side of the cylinder that extends it since the rod is taking up space in the other side, it "overpowers" the rod side and lets the cylinder extend-thereby dumping the bucket.

Since both sides of the cylinder are now "pressurized", the air pocket can not develop, eliminating the "floppy" bucket.

One other added bonus is that the bucket actually dumps faster due to the higher flow rate required to do all this, that's why it's referred to as "fast dump" sometimes.

Source:
http://www.greentractortalk.com/forums/hydraulics/749-what-regen-regenerative-function-front-end-loader-valve-read-here-%3b.html