Need help in understanding Rolling Hash computation in constant time for Rabin-Karp Implementation

I've been trying to implement Rabin-Karp algorithm in Java. I have hard time computing the rolling hash value in constant time. I've found one implementation at http://algs4.cs.princeton.edu/53substring/RabinKarp.java.html. Still I could not get how these two lines work.

txtHash = (txtHash + Q - RM*txt.charAt(i-M) % Q) % Q;
txtHash = (txtHash*R + txt.charAt(i)) % Q;  

I looked at couple of articles on modular arithmetic but no article could able to penetrate my thick skull. Please give some pointers to understand this.