Sony PCG-F540 Primary User Manual page 61

The weight saver protects the multipurpose drive bay when it is empty.
If another device is in the multipurpose drive bay, remove the device.
See or "To remove the second battery pack" on page 48.
Fold out the hinged arm of the weight saver and then push on the
hinged arm to lock it into place. Slide the weight saver in the
direction of the arrow until it clicks.
Manually lock the weight saver's Release lever.