Kinda hoping you weren't gonna ask, because when I explain it, it's gonna sound totally bogus.
Rescripting is where you select a flashback that's giving you grief, and you work with your T to write a new ending. Some people imagine being saved by knight in shjning armour, growing wings and flying away, beating the crap out of their abuser. I'm a pretty literal sort of person, so mine had to be believable- so I imagined someone cottoning on to what was going on, calling the cops, and the police charge in and my abuser gets hauled away.
Once you've come up with the script, the hard part starts. Every time you start having that flashback from that point, you force your brain to run through the rescripted version, instead of what actually happened. It only works if you make sure you do it every single time for that flashback - if they start slipping through the net, it doesn't work.
The basic psychology around it is you're teaching your brain that this is a memory that you don't have to be frightened of any more.
Like I said, sounds bogus, and it's a lot harder to commit to than it sounds. Your brain actually has a lot invested in sticking with the old abuse circuit that it keeps running.
That said, I was a non-believer at first, but flashbacks are actually nothing like the problem that they used to be for me.
It's one of a tonne of different strategies that T's have up their sleeve, so if your T suggests something that sounds that bogus, keep an open mind;)