When there is enough competitors they will divide the times into sections. Sometimes and most comonly by one second. Sometimes by .5 seconds, etc.
If the timd divider was one second and the fastest time was a 15 second run, then the 1D would be 15 seconds, the 2D 16 seconds, 3D 17 seconds, 4D 18 seconds.
So if I ran a 15.3, that would be a "1D run."
Pssh.I didn't pick up the wrong lead
It's called a counter canter...
...A very advanced maneuver.