Just what it says in the 'summary'.

Easily reproducable: make your weight limit borderline: picking up one spellbook will move you to Burdened/Strained/Strained!/Overburdened. Read the spellbook. While you are in the process of reading it (i.e. while the message is on the screen), your burden will go down until you have finished. Then your burden goes back up to what it would be with the spellbook.