Problem solved. I thought it was something to do with the brakes.
The correct answer is indeed a bell! :ffsparr: I've never had a bell that you operated upwards and downwards before, only horizontally!! Well done to all those that posted the correct answer.
I did say the question was from...
Question from Mr. Thicko here;
What is the black round thing on my Trek hybrid bike handlebars? I've just bought the bike and noticed it after I got home! I thought I'd ask the experts on here rather than phoning the shop!
I welcome the usual comedy answers along with hopefully a sensible one...