Premining documentation?
Premining just means you can mine at your own at a very low difficulty before normal users have connected to your network. The size of the reward is determined by the protocoll so you cant change it until the procotol allows it (For Bitcoin it halfs ever 210000 Block). 

For example:

You can mine with a few hashs per second for a week. For bitcoin you get  a block every 10 minutes getting a reward of 50 BTC. This will give you 1008 Blocks and 50400 Coins. If you now  let other people connect they will start mining and the difficulty will increase.