The idea is to arrange your briquettes or charcoal so that only a small amount is lit at one time.
This will control the heat that is available for cooking
Lets have a look at the FUSE in a standard Weber Kettle.
Here we are making a FUSE two briquettes wide with one on top.
If we start the FUSE with 6-8 lit briquettes, this structure will provide roughly 110-120°C (225-248°F) perfect for Low and Slow cooking.
If you start with more lit fuel the temperature will be hotter and the fuse will run slightly faster. Its this initial lit fuel dose that is important
You can also add extra fuel to the FUSE or change the structure but this will not have a huge effect on the heat or speed. Making the FUSE with additional briquettes simply means the FUSE can be shorter.
If you are confused by which Charcoal or Briquette fuel to use for what, read this fact sheet.