Since the RetroTink does not come with instructions, I thought I would provide the basics.
Looking at the device with the two buttons on the left, the bottom one changes your input. (Only use one at a time).
Here is the order of presses: Component, S-Video and Composite.
The top button changes the features. By default, the “line doubling is on. Press the button once and it smooths the picture. Press it again and the line doubling is disabled and the device is in simple “pass-through” mode.