Rotate your bezel counter-clockwise until the triangle faces the minute hand.
Check your watch at any later time. The difference between the position of the triangle and the position of the minute hand will tell you how much time went by.
For safety reasons when diving, the bezel of your TUDOR diving watch can only rotate counter-clockwise. This is to prevent an accidental movement of the bezel leading the diver to think he as more bottom time ahead of him than actually is available.