Manual Calibration button becomes available if the calibration preset has been successfully calibrated. It allows one to manually adjust the binding to sensors and their locations relative to the bones of the skeleton.
To start manual calibration, choose skeletal mesh, then select saved calibration preset and press Manual Calibration button.
The program will switch into the manual calibration mode. You will see the character model with trackers. Hold the Left Mouse Button and use WASD buttons on the keyboard to move within the viewport. Click the Left Mouse Button on any tracker to select it.
Use the drop-down list at the tools panel to change the skeletal bone associated with this tracker.
Switch between position control and orientation control for a tracker by pressing Translation Tool/Rotation Tool buttons respectively.
Press Reset button to reset the calibration. Press Finish button to save changes.
Next: read how to restore calibration in a new session.