Using clip-path for background transition

I need somebody to help me doing this background transition Kevin has in his webpage: https://www.flexboxsimplified.com/ I did something, but it's horrid. I know its using clip-path, but I can't get to understand the logic behind it >_< frustration built up, and I can't progress anymore. Failure attempt: https://codepen.io/myntsu/pen/vYrgNjM (it needs to be positioned absolute, just for the record)