Next focus on pulling up on the pedal stroke (the back of the square) using your hip flexors at the front (bringing your knee towards your chest), and your hamstrings at the back of your thigh (pulling your heel up to your buttock). It may be easier to concentrate on each of these two muscle groups separately, before thinking of the two working in tandem together at the front of the hip and back of the thigh. After a minute or so focussing on this upstroke, change into an easy gear and spin out to relax.
Finally, focus on pushing across the top of the pedalling circle (or top of the square), kicking your heel forwards and using your glute (buttock) muscles to do this. This phase of the pedal stroke is perhaps the most difficult to get a sense of but helps you transition smoothly from the up to the down stroke, and facilitates the use of these most powerful hip muscles. After a minute or so, change into an easy gear and spin out to relax.
To complete the practice, use your ‘working gear’ and try to integrate all four sides of the square into a circle.