Hashing function in custom hashmap

Question

I was reading an article about creating a custom hashmap. Couldn't understand their getSupplementalHash() function implementation. It goes like this:

    h ^= (h >>> 20) ^ (h >>> 12);
    return h ^ (h >>> 7) ^ (h >>> 4);

Can anyone explain to me how this works?

Ref: https://tekmarathon.com/2013/03/11/creating-our-own-hashmap-in-java/