As John suggested above, they are indicating that you need to pre-load the indicator. That way, it can read both positive and negative values. Physically locate your indicator to read 2 mm (or whatever the inch equivalent is) by pushing it slightly against the hub, and then push and pull on it as you mentioned. In your head, you can re-zero the indicator at 2 mm, and then simply read values from there.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).