Its that thing you are meant to look at but cant take your eyes of the road or bike ahead! I'll ask Julie to do one for you next time, I never see mine!
I changed mine this year to GPS laptimer due to ease of setting up rather than blanking for other peoples beacons. Usefull for seeing if your pace is dropping off, but just found out if there are people close enough to you then I dont bother ever looking at it more interested in beating them than the clock.
I was convinced I needed one when I didn't have one, and after spending £200 on a GPS one, I discovered that I am usually too busy to look at it till I have got off my bike after the session.