Flame weeding is used to kill all of the newly sprouted weeds before they get large enough to compete with the planted vegetables. I am flaming the weeds in a bed that was planted a week ago. The lettuce seeds have not yet germinated and they are safe from the torch under the cover of soil.
I bought an inexpensive propane torch at a hardware store and every time I let go of the thumb trigger, the flame would go out. The torch is designed so that you open a flame adjustment valve on the handle to set the maximum flow of propane but the propane will not pass to the end of the torch until you push a thumb trigger. When you let go of the trigger, the torch is extinguished.
I pulled the thumb trigger apart and put a scar in the valve assembly so just a little propane will constantly leak past the trigger when the flame adjustment valve is open. Now when I let go of the thumb trigger the torch stays lit and I don't have to reach for the striker at the end of each row.
When the lettuce finally pokes through the soil it will be greeted by a weed free seed bed.