Officers tell a driver they can smell intoxicants - alcohol has no smell - and then explain how the breathalyser works.
The roadside breathalyser works on a traffic light system and takes about 40 seconds to get a result.
Green means the driver has had nothing to drink.
If it goes to amber it means the machine is `thinking'.
It has detected alcohol and is assessing if it is over the legal limit. It will then go to green or red.
If a person is over the limit - and well over - it will shoot straight to red.
Red means instant arrest.
Once there drivers blow into another breathalyser machine which gives the reading that will be used in court.
They also have the option to give a blood sample instead.
Police advise people not to touch a drop of booze if they are driving for no-one knows exactly how quickly a person's body absorbs alcohol into the system. Everyone is different.
But never drive the morning after a heavy drinking session as you're likely to be well over.
Anyone with information about a drink driver should phone West Yorkshire Police on 0845 6060606 or Crimestoppers on 0800 555 111.
If you spot the person driving or about to set off then phone 999.
Ideally give the drinker's name and address, the make, model, colour and registration number of the car, where they normally drink and the time they set off to go home.
In short, the more information the better.