The B-axis simultaneous option can be used to cut parts such as this one:
Using a vertical tool, the tool collides with the part as shown:
Using a neutral tool with a fixed B axis, the tool copes with the undercut part of this example:
However, it collides further along:
Using the simultaneous B-axis option, the tool can cut the entire part without colliding, because it can move in the B-axis as it cuts: