What they're telling you to do is to prime the battery by trickle charging it. The problem is that you can't and shouldn't trickle charge a Lithium based battery. Once a Lithium based battery is full, it just does a small topping up charge periodically. The priming was only necessary for Nickel based batteries. Once the charger light went off, they would generally just go into trickle charge mode. Lithium batteries tend to react badly if you overcharge them.

No idea why it is still generally suggested though.