It is the first step in a two-step process to transfer the part from the main spindle to a subspindle.
The Subspindle Grab command drives the subspindle up to the main spindle at the clearance height, feeds to a depth, and clamps onto the part. Next, you usually perform a Turning Part operation to cut off the part.
Turning subspindle at the Feed Plane.
Turning subspindle at the Chuck Plane clamping onto the part.