Tip: breaking circles
I was a bit confused about how to use BREAK on circles and leave the bit that I wanted to leave. It's alluded to in the help (When you break a circle, CC3 keeps the portion of the circle you selected at the "entity to break:" prompt), but this diagram might make it clearer.
EDIT: Sorry, the images and text are a bit small because I made it into 1 image, but if you click on it you can zoom.
EDIT 2: Actually, I think the diagram makes it more confusing. The most important part to remember is the side of the circle you click on will be the bit that's kept when the command is executed. It's a command where it matters where you click on the entity when making the selection when it comes to circles.