It's not the position of the ram, but the direction of last movement
The square ratchet (otherwise known as the "little black square thingy") slides up and down the indexing rod, forced by the indexing arm (the square ratchet is trapped inside of it). Since there is some drag, the square ratchet is forced against the interior underside of the top of the indexing arm when the ram movement is down. The underside is where the ratchet notches are. If the square ratchet is engaged with those notches, and you rotate the turret (and the indexing rod) in the backwards direction, you will feel resistance. If you force it, you will break the square ratchet. It's that simple.
Let me simplify it further.
It does not matter where the indexing arm is located. It only matters where the square ratchet is in relation to those notches. But you cannot SEE the position of the square ratchet, so you have to rely on remembering the direction of travel of the ram, the primary determining factor.
If the last movement of the ram was downward, the square ratchet will be up and engaged with the notches. If it was upward, the square ratchet will be down and not be engaged. If not engaged, no breakage. If engaged, you can break the square ratchet, almost guaranteed. Do not pass go, pay 50 cents and start over.
I will repeat. The stationary position of the ram is irrelevant. The position of the hidden square ratchet is relevant. The position of the square ratchet is determined by the direction of the most recent movement of the ram.
Most recent movement up, disengaged; breakage impossible. Down, engaged; breakage possible.
In mnemonic form:
Most recent movement up, OK.
Down, not ok
Remember the signal from the Roman Coliseum:
Thumbs up, your square ratchet lives.
Thumbs down, your square ratchet dies.
Lost Sheep