There isnt really too much to the Safe Z.
What it does is when you press Go To Z button it will raise Z first then move X and Y then finally lower Z.
There are 3 options for you Safe Z height.
Machine Coords
Work Coords
Incremental height
If using Machine Coords then the Z will raise to whatever you have set before X and Y will move. So for example if you had -1 set then it would move to 1 unit from fully up.
If Using Work Coords then it will move to whatever you have entered with respect to your work coords. For example if you had 2 set then it would move to 2 units above your work coords zero.
Incremental will make it move up the amount you have set from any position you are currently at. For example if you are currently at Z zero and you have a value of 2 then it will move to Z 2, If you are at Z10 when you press Go toZ it will then move to Z12.
Hood