From the Glossary: Flatlining: Many NoFap® users report one or more periods of zero libido during their streak, especially in the 2-6 week period. Transitioning from an overexcited, always-eager libido to none at all can be disconcerting and even scary for the experienced masturbator, but many Fapstronauts report that it is only a phase in the reboot and will pass. Basically, your body is adjusting to the changes in chemistry. It's a difficult time for a rebooter, because it will cause anxiety. "Oh crap?! What happened? Am I going to be impotent forever?"
We can tell you all you want (which I don't see why you couldn't simply google it), but it's more of a feeling. You'll know when you hit it.
1) Porn -> Large amount of Dop. 2) Porn everyday - Brain gets alot of dop. 3) Brain - Scared of such large amount of chemical. Reduces the receptors As a result you will seek more porn to get same pleasure. 4) Now NoFap - Low triggers 5) Low triggers - Low dop. 6) Brain - Whats happening? I need dop. Withdrawl symptoms. 7) No dop. - Dead dick. This is flatline. During flatline your brain will try to revert to original state.