The mechanism is a Penn State Industries' TrimlineŠ pencil.
Extend the pencil lead by pressing on the top button.
Retract the pencil lead by holding the top button down while pushing the lead back into the pencil.
Uncover the eraser by pulling the top button off.
Replacing the Pencil Lead
Pull the top button off.
Pull the eraser out of the fill tube.
Drop one or more 0.7mm pencil leads into the fill tube. Replace the eraser. Replace the top button. Push the top button to start advancing the pencil lead.
Extend the pencil lead by pressing on the top button.
Retract the pencil lead by holding the top button down while pushing the lead back into the pencil.
Uncover the eraser by pulling the top button off.
Pull the top button off.
Pull the eraser out of the fill tube.
Drop one or more 0.7mm pencil leads into the fill tube. Replace the eraser. Replace the top button. Push the top button to start advancing the pencil lead.