I think what he is talking about is that during a run or when being marched he might get called out to lead and he should have a few cadences on hand for that situation.
When running as a platoon at double time march the cadence is 180 beats per minute, as that is how fast a pace you should be maintaining.
Do you know that commercial where the group of people drinking that starbucks coffee thingamajigger are all yelling in the street, that is about the pace of a running cadence.
When marching as a platoon the cadence is 120 bpm, noticably slower, which is why you never mix a running cadence with a marching cadence without altering it a lot.
The best way to match a cadence with your platoon is to remember that beats correspond to your steps.
So when I say your left, you say your right...
There was a great site that explained all of this, but alas it has been killed be the ravages of time...