Maynarding is the act of transferring workers from an existing mining base to a new expansion in order for it to a) start providing the player with income immediately and b) increase overall mining efficiency. Experiments have noted it is not worthwhile to transfer less than 6 workers to a new base, due to the time loss in harvesting inactivity while the workers are in transit.
Maynarding received its name from a US Terran player called Maynard who is said to have popularized the technique in the early days of StarCraft.
Number of Workers
The optimal number of workers to transfer in order to achieve maximum mining efficiency depends on many factors (race, total number of workers etc.) and no absolute rule exists. A basic study however can be found here . It recommends an identical worker/patch ratio at the initial and the target location.
Generally, players should never transfer more than half the workers from the initial base and in case of a rather fast expansion(i.e. before the number of workers at the main base is greater than two times the number of mineral patches), the number of workers to be transferred should be such that the main base is left with at least one worker per mineral patch plus 3 for gathering gas.
Cloning the Maynarded workers to different mineral patches before they reach the new expansion is commonly used to further increase the speed at which it provides optimal income.