The burner and the vent — the only two controls
One makes the balloon go up. One makes it go down. There is no third control, and the absence is the whole design.
A hot air balloon has two controls. That is not a simplification for beginners; that is the complete list.
The burner
A propane burner mounted above the basket, pointing up into the envelope’s mouth. Pull the blast valve and it produces a column of flame a couple of metres long and a great deal of noise. Let go and it stops.
Burners are usually doubled, in two independent systems fed from separate tanks, because the consequence of a burner that will not light is not “the flight ends” but “the flight ends downwards”.
There is also a quieter, less efficient setting used over livestock and near houses — burning without the roar. Ballooning has a lot of etiquette like this, because balloons pass low over other people’s property all the time.
The vent
At the very top of the envelope is a circular panel, held closed by the pressure of the hot air inside and by a line running down to the basket. Pull the line and the panel opens; hot air escapes from the crown; the balloon cools and descends. Let go and the pressure seals it again.
Because it is shaped like a parachute pushed up into the fabric from below, it is usually called the parachute vent. On landing, pulling it all the way down deflates the envelope on purpose, which is what stops a balloon being dragged across a field by a wind that is still perfectly happy to fly it.
Burner: up. Vent: down. Neither of them turns the balloon, because nothing on a balloon can turn it. Direction comes from somewhere else entirely — see why a balloon has no steering wheel.
What “letting go” means
Both controls are momentary. Neither has a setting you leave it at. So the resting state of a balloon is no input, and a pilot who does nothing is not holding an altitude — they are cooling, and slowly going down.
Holding a height therefore means burning a little, regularly, for as long as you want to be at that height. It is less like setting a dial and more like keeping a kettle just off the boil.