dedicated to the discussion of the chinese internal martial arts of xingyiquan, baguazhang, taijiquan, related arts, and anything else best discussed over a bottle of rum
Downward force or pressure is one of several expressions of 6-directional force. We use the concept of a "sphere" to represent our body structure, and it's the manipulations of that structure that gives the impression of "rolling that sphere" in various directions to cause an opponent to be projected laterally/horizontally, or vertically upward or downward. The process to create the 6-directional force is the same for all three; it's just the way that we manipulate our "shape" internally that determines the way we will affect the opponent.
In the Japanese internal arts, the process of applying downward or compressing force is called "aiki-sage" ("sage" is pronounced "SAH-gei"). It is delivered both directly to the opponent's center of mass, which causes him to buckle at the waist, and through his joints, which compresses the vertebrae and diaphagm (which controls his breath). Applications are varied. Here's a short clip of aikijujutsu where you will see downward force applied: