Tappets control the board position or the timing of the opening and closing of the warp. Combining the threading of the warp through the heddles with the movement of the boards will determine the pattern.
Most looms weaving Harris Tweed have the tappets set to 1&2,2&3,3&4,1&4 ie producing a standard 4 tappets weave, although other variations of up to 24 tappets can be used. Changing the tappets is a slow and time consuming task and makes the loom harder to pedal and the tweed longer to weave.
Adding additional tappets creates interesting patterns and designs.