unidirectional bezel = provides divers with alternative source of elapsed time for the current dive.

From google;
"Why a unidirectional elapsed timing bezel is so important. While diving the actual time is irrelevant, what is important is the time remaining for your dive. The elapsed timing bezel is lined up with the minute hand on your watch prior to entry and thus letting you know how much time you have been submerged. If the bezel is bi-directional, a simple knock can dislodge it and cause you to overstay your bottom time beyond no-decompression limits, which can be extremely DANGEROUS. This is also the reason why a dive computer should be your primary source of information."

Hope this helps..