How to understand difficult irules

I am assuming you have completed reading the iRule 101: https://devcentral.f5.com/wiki/iRules.iRules101.ashx

Quite a few of the iRules use the event HTTP_REQUEST. Understand the different scenarios to use it.

Try to read and understand the 20linesorless iRule series: https://devcentral.f5.com/wiki/iRules.20LinesOrLess.ashx