• the control lever is pushed down.
• the lift arms fall, until-
• the drag on the plough creates enough pressure on the top link to compress the spring, which then pushes the control fork pivot point forward. The valve is then closed, and further descent of the lift arms is prevented. The plough continues at this depth, until-
• further resistance is met by the plough
• the control fork pivot point is pushed forward
• oil is pumped to the ram, lifting the lift arms.

The animation doesn't show the effect of a reduction in drag: that would let the top link, and the control fork, move back, allowing oil to be drained from the ram again.