What is the 2T/4T feature on a welder?

The 2T/4T feature is where the welder can choose to weld in:

2T Mode

Press the gun/torch trigger to weld and release to stop.

or

4T Mode

Press and release the gun/torch trigger to start, weld without holding the trigger on and stop by pressing and releasing the trigger again. This is particularly useful when doing long weld runs.