When pressed, it marks the paper. This is done for creating a record as an evidence of a action like a fix or a a happening like a light coming on the beam.
Paper Speed Control (Fast / Slow) of Echo sounder
Faster the paper clear the record trace which is essential to appreciate the shape of the sea bottom. This visual display is a great help when sounding is used to fix a ship.
Gain & Sensitivity of echo sounder
To make echoes darker on the paper. Best set to when the echoes become just about discernible.
Dimmer of echo sounder
Keep to optimum settings or else it will affect your night vision.