The old tachograph had a small razor sharp blade that put a small slice on the edge of the paper disk when you closed the door. Or opened it. And the slits were on the disk OD and cut a time record. So you would normally end your day and take out the disk and have two slits on the disk at your start and stop times. But if you opened it during a trip so it wouldn't record you'd have two slits, but the second one would not correspond to your end time. Many drivers tried to out-smart them, but to no avail.
The Jalopy Journal
