It is an application of the slider-crank mechanism. The seat plays role of a connection rod. When folding, the contact between the seat rear bar with the back tubes creates a sliding joint. Upper crossbar of the front legs is used as a stopper when unfolding.
Lift the back side of the seat to fold.

The list of all videos in channel thang010146 (uploaded till 31 Oct. 2017) including pictures, brief descriptions and YouTube links (2700 Animated Mechanical Mechanisms) now is available.The list consists of 4 parts:
Part 1: Transmission of continuous rotation
Part 2: Other kinds of motion transmission
Part 3: Mechanisms of specific purposes
Part 4: Mechanisms for various industries