Here are some already-existing threads about this:
- https://longhaircareforum.com/threads/reversing-it-deep-conditioning-then-shampooing.123295/
- https://longhaircareforum.com/threads/dc-before-washing.437616
- https://longhaircareforum.com/threads/does-it-make-any-sense-to-deep-condition-b4-wash.345843
- https://longhaircareforum.com/threads/dc-on-dry-hair-before-washing.400804
Perhaps these links about the condition-wash-condition method (the CWC method, also known as the CSC, or condition-shampoo-condition method) will shed some light:
- http://longhairsecrets.com/cleansing-hair-condition-wash-condition-cwc/
- http://www.venusianglow.com/2013/04/washing-hair-with-condition-shampoo.html
A couple of comments jumped out at me. People feel that DCs deeply penetrate, so the wash afterwards that simply washes the outer layer of the strands does not work against the benefits of the just-done DC.
Then, it seems to be similar to the point of pre-pooing: The pre-wash DC gets the hair in good shape so that the post-DC wash does not harm the strands . . . and so that detangling is then done on fortified strands.
I started doing it without even realizing it: I would take my SM deep conditioner that just SINKS into my strands and use it to
safely soften and stretch and strengthen my strands. Sometimes I would steam it in. Then I would apply shampoo on top and wash it all out in the shower. After that I would apply my leave-in and styler.
If you had told me I was DCing, then washing, then (leave-in) conditioning, I would have looked at you like you were confused.
I would have said, "No, I'm just kind of doing what I call a FAST prepoo instead of an overnight one, thank you very much."
ETA: I just wanted to say, I have no idea whether shampooing reverses the effects of DCing.