What is ERP
ERP stands for 'Exposure Response Prevention Therapy'.
The client is repeatedly exposed to the thing which causes him anxiety, until he experiences a marked relief of symptoms. For example, with someone obsessed with hygiene, he is exposed to germs by touching a toilet seat, and has to refrain from washing. This causes him massive fears, but after 15-30 minutes, a decrease of anxiety occurs.
Not reacting is key, learn to accept obsessions and not act desperately on it.
Why ERP
Traditional talk therapy isn't good for compulsions, because OCD is not something that can be rationalised or discussed.
Studies show that OCD sufferers have less gray matter in areas that suppress responses. So you can't just tell an OCD patient to not worry, it's like telling someone with Asthma to stop having trouble breathing.
CBT is also not effective for OCD, because CBT is all about challenging obsessions, but this can actually reinforce the belief that these thoughts are significant.
Dealing with Pain
ERP can be considered messed up, because it's making people go through pain. However distress is inescapable, most clinicians agree that short-term pain is way better than long-term anguish. And so most therapists are willing to make a patient's pain worse for a time, so they can eventually get better.
OCD patients will have irrational thoughts (like thinking about harming or killing someone), and so write an excessive amount of notes on how good of a person he actually is.
It's not just about having quick irrational thoughts. We all have thoughts we wish we don't have, the difference is that people with compulsions are stuck with them.
However, ERP's treatment is not limited to 'significant' thoughts. There's no evidence that someone with more 'disturbing obsessions' (raping, killing others) suffers more harm than someone whose thoughts are 'trivial' (cleanliness, obsessive notetaking etc.)
In ERP, there's no such thing as an obsession that's trivial or distress that isn't valid.
Source:
https://www.treatmyocd.com/blog/what-exactly-is-exposure-and-response-prevention-erp/