Bryan
It's not a bad idea to turn off the chassis main disconnect before working on the cables as there is a large current capability on those and a mis step and resulting short would be a bad thing.
Remember the engine starter cable is not turned off by the disconnect and will remain live with the disconnect turned off.
A volt drop test is relatively simple .
The concept assumes that in a loaded circuit the applied voltage is consumed by the load it self say a starter motor but in reality there are other things in the circuit which can consume some of the voltage through resistance things like solenoid contacts or cable connectors.
Say we apply 12 volts to a starter circuit and we connect one voltmeter lead to one side of the starter solenoid and the other lead to the other side. with the solenoid closed or making contact there should be a zero volt reading on your meter as the contacts should be passing the current and voltage straight through but in reality there will be some resistance in the connections in the solenoid and with use these contact's get dirty causing higher and higher resistance until they finally they won't pass current any more
With the meter connected as the contact resistance increases the the voltage drop across the connect increases this voltage drop will show on your meter 0 volts would be a perfect connection and anything over 1 volt would be a poor connection.
REMEMBER the circuit has to be under load to make these tests.
If you took the starter circuit and measured the volt drop for each of the cable connections and the starter itself the total of all the volt drops should add up to the applied voltage.
Each of your battery cable connections will be the same if a connection has any resistance under load it will generate heat and the voltage will drop across it .
Here is a video link that may explain volt drop testing better than I could.
https://youtu.be/APLic9F7ZsAEric