Whats the purpose of the different Kriya Yoga techniques?

[MM1988]

Whats the purpose of Maha Mudra, Kriya Pranayama and Yoni Mudra?

I only have the Santata Gamana books which only talk about how to do the techniques.

But what is each of these technique really doing or what is their goal and how do they work?

I guess you dont have to know to practice it but Im interrested.