This allows you to control the amperage when actually welding which is very beneficial especially when welding aluminium or exotic steels / alloys. It acts like a car throttle controlling the amperage.
You simply unplug the torch switch plug from machine and plug in the foot pedal.
You then set the main control amperage on machine front panel to the maximum you wish the foot pedal to go to on a maximum depression. So if you set the main amperage control to 100amps, when fully depressed the foot pedal will give 100amps. If set to 40 amps, it will go from minimum amps to 40amps allowing fine control of amperage.
Note: Some competitors foot pedals have the maximum amperage knob on the side of the foot pedal, so it can be hard to see what the maximum has been set and also easily knocked as on floor, this is quite an old fashioned way (we changed to machine control of foot pedal max amps over 17 years ago)
Once a weld pool is formed and you are moving along the weld as the heat transfers into the workpiece you may require less amperage as the weld pool width / penetration will get too hot, reducing the welding current with foot pedal will stop this happening.
Over a 300mm long weld, you may need 100amps at the beginning and only 80 amps at the end of weld. Using a foot pedal will give you a uniform weld bead size.
It also allows you to back off the heat from weld if you need to reposition torch / get new filler rod but don't want to stop the weld I.E When doing pipework etc which requires one continuous weld.
Most experienced tig welders prefer to use a foot pedal especially on aluminium and exotic steels / alloys, unless the welding is not being done at a workbench i.e. working on a car etc.