Sony PCG-F420 Primary User Manual page 51

The weight saver protects the multipurpose bay when it is empty.
If another device is in the multipurpose bay, remove the device. See
"To remove the floppy disk drive" on page 25 or "To remove the
second battery pack" on page 40.
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.