The "start late, leave early" stuff basically means - leave out the boring bits.
Say your scene was the investigation of a crime scene. Parts of the scene:
1- the call comes into the police station
2- a detective is assigned
3- they travel to the scene
4- they start investigating
5- they find some evidence
6- they bag the evidence
7- they travel back to the station
The advice given basically means cut straight to point 4 and exit the scene at point 5. All the rest is implied, and only needs to be shown if something relevant to the plot happens in those parts. And even if it does, the same advice applies to that "micro" scene.
Let's say on the way back to the station, the detective is ambushed by the bad guys. Don't go through all the getting into the car and driving along - cut to the moment before the ambush happens. Then leave after the ambush is finished - don't show them limping back to the station.
Basically, keep all the good bits and leave the boring stuff out.