The manual was always vague on the real use.
Think of it like this,
At 1, the readout tells us to the inch when an object moves. At 2, the resolution is now only 2 inches.
At a positive value, it will readout constantly even if the object has not changed distance, i.e. it will constantly update 5'5" as it senses. (depending on how you set your sensitivity.)
At the -1 or -2 value, it will not change the readout until there is a definite change in the subject distance, (i.e. movement starts to happen.) I personally do not like this mode as its not very accurate and the display can sometimes seem like it's stuck. I'm used to the jumpy numbers.
TL:DR the minus values are less jumpy on the display and will only change when there's definite movement.