How would the driver release steam on a 3F 'Jinty'?

[rdlover777]

I've been watching a few videos of steams engines releasing steam from under the wheels and i was wondering how a 3F would do it?

Edit: Just after i posted this, my grand dad came to visit and i asked him. He said its a cylinder drain.

 

[ac6000cw]

Cylinder drains are vitally important on a steam loco - they allow water that might have condensed out and accumulated in the cylinders while the loco is stationary to drain out (before it starts moving).

Unlike steam, water is not compressible, so if significant water is trapped between the piston and the end of the cylinder when the piston reaches the end it can cause serious damage - it's like the piston hitting a brick wall.