Ian Marples, President of Guardian Interlock Systems - the only Canadian company that makes car breathalysers, says it was obvious people would try to fool the device by having somebody else give a breath sample. So they added a second step. You have to hum and blow at the same time. That is tough enough to learn let alone teach someone else to do on the spot. You then blow into the device again and it processes the sample and tells you if you can start the car.
Driver gets into the car.
Blows and hums into the device. Sample is taken and analysed. If everything checks out, the car can be started. And even after you're driving you're not home free. The ignition interlock system will demand that you do a retest at random intervals.