Here is a blog that I wrote sometimes back on York Antwerp rules and general average. I hope this could be of some help.

York Antwerp Rules and General Average, Everything you Need to Know

If you have any other specific question on this subject, do not hesitate to ask.

In fact General Average originated much before Marine Insurance. There are evidences that GA existed during Justinian Code. In simple words, GA is based on the principle of equity. In a lighter vein , I quote the example of sharing system in college mess; those who eat profusely and a poor eater has to share equally. So GA is where losses are incurred deliberately and reasonably for the benefit of saving the cargo owners, Ship owner and Freight earner from losses, such losses are made good by contribution from all those whose interests are saved.