If you've been told you need to use an ignition interlock system in Maryland, you blow into the device just like you would a standard breath test, and it measures the alcohol in your breath. If you pass, you can start your car. If you get a failed reading, the device won't let you start the vehicle until you pass.
However, there are a few things you should know about failed readings. First off, the device itself cannot become defective and falsely detect alcohol. Defective devices never function in this way. Some people try to claim they have defective systems, but those will never give you a failed reading when you really should have passed.
The next thing to know is that some items in your home may cause you to fail, even if you have not consumed alcohol. For example, you may fail if you just used mouthwash. The devices can also fail because of cough medicine, perfume and things of this nature. Avoid these things if you have to drive, or at least rinse your mouth out and wait a few minutes, after which the system should work.
Finally, you should know that the device may need you to do a re-test at certain times. You have to do it. Turning off the car is not an option. If you do, the horn sounds and the device records your actions. Always do the re-test if the device determines it is needed.
In addition to knowing how to use these devices properly, you need to know all of your legal rights after you have been arrested for drunk driving.
Source: Maryland Motor Vehicle Administration, "Ignition Interlock Program," accessed July 16, 2015