You can cut sides with it with a special corner, not for example only the x and the y axis are getting a diagonal cut, also the z axis is getting a diagonal cut together with the other 2 axes.
Let's try to cut this point of the brush
First cut the cornor like this using 2 clip points, this looks familiar isn't it.
I made some yellow points and lines in the 3d view, so you will see where the location of the points in 3d will be, and how it will be cutted.
Now to finish it, we need to put a point some where in the map, but how? The 3th point must between the 1st and 2nd, and it also must be above the upside of the brush. Well let's try it to hit the xyz buttom (market with the red arrow) first, so you will get the front view.
Now place the 3th point (market with the green circle) above the brush, then slide with it untill you will get what u want, the 3d view would be really helpfull to do this. dude... it works..
Let's have a view from an other side,

cool so hard to find out, so easy to do it.
At0miC