Laser and Z Height question

I just recieved my JTech 14W plus air assist, but have not installed it yet. I will be using the laser add on for VCarve Pro. I have read that if you are cutting thru materials, it is best to set the focus at the bottom surface of the material being cut.

Here is my question… When doing laser work, does the Z stay fixed during the “cuts”? I would assume so, but curious why the Z couldnt be raised so the focus is at the surface on pass one, and then drops down to the bottm surface as your cut proceeds. (Provided you dont crash into the material surface of course).

laser stays the same height (not using vcarve but lightburn)