In terms of martial practice, the trigrams correspond to both the "changes of palm", and the "mother palms".
The 8 Mother Palms are a method of power generation training. Each trigram relates to an expression of power, such as "Heaven Palm" relates the forward and upward power.
The Tai Chi classics mention Ba Jin (8 forces) each of which is identified with a different triagram of the Ba Gua. These forces are based on the direction in which the type of Jin (force) is usually applied.