The spot weld function allows you to set a "time" 1 to 10. Then when you pull trigger it will weld for just that time. Then you have to release trigger to start time again. Allows you therefor to set welder to weld for a set time instead of guessing. I have same function but just use experience of welder to do same, saves going back to welder and having to turn back to normal.
As there is no seperate switch on yours I would guess you have to select 0 to stop the spot weld function.