cPanel would probably use the same method to change the password as you do through SSH youself, as many things in WHM is like that.
I dont see how or why WHM would change the password for resellers though if you change the root password.
Dont see a reasoning that WHM would want to do that as it just causes problems with all resellers know having access to their accounts, and I doubt it would make all resellers have the root password

hehe