When you're dancing the LF back rock as man, you have what amounts to two footwork choices: (1) Release the heel of the front foot as usual, then replace to a ball-heel, or (2) release the toe of the front foot, and then replace to heel.
If you were wondering about a third choice, don't. Releasing the heel of the RF, then quickly flipping up the toe to replace to a heel is awkward and unnecessary. While there are most certainly circumstances where we might begin a step with the toe in contact with the floor, and then flip up the toe at the last moment to take a heel lead (first step of promenade being a fine example), the positions and timing of that second step of the rock make it a poor candidate for such a foot action.
So assuming we all agree that's not an option, we are left with the first two. Now the ISTD specifically wants you to replace to a heel, suggesting that choice #2 is in fact the correct one. So I would recommend using that for your next exam, and probably also a competition. But I think you would be forgiven if, in other situations, you were to take that step in a manner consistent with all other LF back steps with left side leading in Tango, thereby replacing to a ball-heel. Me? I actually prefer it.
Regards,
Jonathan