What's hard to understand about the fork?
Bitcoin is an equation. A mathematical specification of how much bitcoins are there at any given moment of time, how new blocks are added to the chain, which sequence of blocks is accepted to be authoritative, etc. This is why it is a work of genius. An equation can't be hacked, unless you can challenge underlying mathematical assumptions, which is somewhat hard to do.
However, you can't run an equation on its own. You need an implementation, which actually runs calculations in the equation ("mining", verifying transactions etc). Technically, anybody can write their own implementation of the Bitcoin equation for running the blockchain, substituting their own parameters (e.g. maximum block size, how often the blocks are generated, the reward amount etc.) I can write a Bitcoin client today where new blocks will be attached to the chain the way I want it to. Giving me free money, for example. Or increasing the block size to shovel more transactions in. However, the Bitcoin network consists of many thousands of clients (A.K.A. "full node" clients). They have to accept my blocks to consider them valid, and sign the acceptance, and they won't accept anything that is not hardcoded in their implementation. tl;dr I (or anybody else) can fork Bitcoin today -- the only idea is that I will have hard time to persuade others to accept my fork. And without acceptance, anything stored or transacted in this fork will have no value.
However, some people manage to persuade others to run their fork as it has some feature some see as valid. As they are accepted by some people, they start to have value (and can be bought and sold by gamblers). However, more often than not, these forks fail to catch traction and deemed to obscurity, and as their user base diminishes, so it their value.