Clipping planes - Shift + right click to place a point. You can place 2 or 3. If you want to get an angle not on the x or y axis, you need to place 3. There are 3 buttons you can hit - one splits the brush and keeps both halves, the other deletes one half completely. There's also a button to flip the clipping plane if you want to keep the other side. Hover your mouse over them to figure out which they are - look for "split" and "carve". I think they're about 1/3 of the way from the right side of the screen.
