The regen process on the 6.4 powerstroke takes place when the DPF starts to clog up. During the process raw fuel is injected into cylinder 7 and 8 on the exhaust stroke. The purpose of this is to burn off all of the soot that the DPF has collected. During this process the exhaust gas temperature (EGT) get up to 1500 degrees, as you may imagine this can reek havoc on your engine and powerstroke performance. It is my opinion that this is the number one reason that the 6.4 is not as reliable as it could be. If the DPF gets clogged it will be a very costly repair. One other way to repair this issue is to delete the DPF and regen process.