You are literally cutting the top bud off and allowing the 2 chutes underneath it to develop as main buds. It redirects all of the growth from the apical meristem to all the bud sites.
Kind of set the plant into panic mode and redirects all the growth.
When you do this tho make sure you do it with feeding.
Autoflowers some people say it is a bad idea to top. I have done it and really like the results but i will only do it once on autos. Hope i helped.