The mixing valve is a small volume chamber where solvent A and solvent B are combined to provide a smooth continuous gradient.
The mixing valve receives solvents A and B as delivered by the proportioning valves, where the two solvents are mixed to form the mobile phase. The mobile phase then enters the pump and is delivered to the column.
This simulation shows a low pressure mixing system, where mixing occurs before the pump. In many LC systems, the solvents are mixed under high pressure (after the pumps). Mobile phase composition is controlled by the relative rates of the two pumps.