If this option is toggled on, the user can define the rotation center prior to rotating the model. If the  button is hit, eta/DYNAFORM prints the following message in the message prompt window:

PICK A LOCATION AS ROTATION CENTER

The user can click anywhere in display window as the rotation center. Then, holding the left mouse button down, move the mouse cursor to rotate the model around the defined center.

If the rotation shortcut key (<CTRL> + left mouse button) is used to rotate the model, the immediate location upon which the mouse cursor is placed is automatically defined as the rotate center. The model is rotated around the center.