The triangle is used to draw or measure the length of a line. In combination with the buttons on the edge of the drawing board it can be used to quickly make lines with an angle of 30, 45, 60 or 90 degrees. The next video shows how this is done in practice.
Important: these videos show a previous version of the TactiPad. The appearance of some components has changed in newer versions of the drawing board, yet the functionality is still the same.