Calls down a MULE, a temporary unit that can mine minerals and repair for the Terran forces. They last 90 seconds of game time, or about 65 seconds on "faster" game speed. If the MULE is cast on a mineral patch it will begin to mine minerals as soon as it spawns. It takes 28 seconds for an orbital command to generate 50 energy, meaning you should calldown new mules about every 30 seconds.
Despite their timed life and Orbital Command energy cost, a MULE has several advantages over an SCV: a MULE costs no supply. MULEs have a mining time 2.05x longer than an SCV, but have the same movement speed and acceleration. MULEs carry 6 times more minerals per trip than an SCV, gathering 30 minerals per trip on both standard and High-Yield patches. Their timed life allows 8 or 9 mining trips which totals 240/270 minerals. This works out to 160/180 minerals per game-minute respectively. Also, MULEs can mine over other SCVs.
MULEs on blue minerals are as effective as slightly less than 4 or 4.5 SCVs, as SCVs average 42-43 minerals per game-minute on blue mineral patches (for two or less workers per patch). On yellow minerals the MULE effectivity drops to about 2.5 or 3 SCVs.
Generally speaking, you should be using your MULE every time your Orbital Command energy gets to 50. An exception to this is if your opponent has gone for fast cloak of any kind, then you may have to save energy for scans. You should call down MULEs on the closest patches with highest minerals, and at the newest expansion, if possible. This will keep your income high throughout the game, and ensure that no patches are mined out prematurely.