Pen dissasembly kit - whats the metal lump for?

Hi all,



Ive bought a pen disassembly kit: http://www.timberbits.com/pen-disassembly-tool.html



The two rods are straight forward enough... put em down the respective hole and give them a bonk with a hammer.... but the large bulby metal thing with the hole through it?



What do I do with that?