It's normal to use a Pu Er knife and gently break the leaves away from the cake.
A Pu Er knife is pretty much like a thick blunt letter opener. At home I usually just use a butter knife. The best way to do it is to try to slip the knife in from the side between the layers of leaves, or from the inside of the back, and then twist the knife slightly to break a small peice away. Use as little force as possible, and do your best not to damage the tea leaves while breaking it away. Also, just break off quarter or dimed size pieces of compressed tea, rather than trying to break it down into really small bits.
here is a pictorial guide on another tea vendor's site on how to break apart a pu er cake:
and another single frame picture:
Another method is the steam method.. In which you put the whole thing on a steamer rack, steam the cake until it loosens up some, and then pull it apart once the leaves are malleable again. Then let it dry, and store it like loose leaf tea. That requires a certain amount of time and labour that chipping it away doesn't.
Hope that helps,