If you have read the page entitled Half-Bridge you will remember that the battery current is being switched on and off 20,000 times per second. Now electricity in wires has the equivalent of 'mass' - it does not like stopping and starting this rapidly! So most controllers have a main capacitor (a bit like a local battery) to compensate for some of the 'inductance' (as the mass effect is called) in the battery and power leads.
In the higher current controllers, this main capacitor is actually doing a lot of work (read - it can get hot). The amount of work it does depends on the layout of your batteries and the length of their wires as well as on the motor current. Indeed, this main capacitor can be a limiting design factor in most the smaller controllers such as our Porter, 1QD, 2QD and similar. So higher current controllers need to be much more complicated designs!
If this capacitor is not present and the leads are long, something else must do the work: the MOSFETs! Controllers without it therefore are slightly less efficient (read - they get hotter quicker) especially if the battery leads are long.
If you are a beginner to electricity and all these amps and volts confuse you, try reading our page Understanding Electricity - an analogy with water.
If you are technical enough to understand circuit diagrams, there is a much more detailed description in our circuits archive.
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