Can anyone tell me how to use this feature...i cant seem to figure it out.

what im tryign to do is this:
i have a wall and want to add a arch mesh inbetween, i dont know if im supposed to do.

Also.....im a noob with the editor, i have never mapped for ut or ut2k3....only q2,q3 so im familiar with all the terms with the editors just not the interface and the differences.

Okay here is a quickie.
1. Highlight the brush you want to Clip.
2. Click on the Brush Clipping Button.
3. CTRL RMB on the Point where you want the clipping to start. You should see a Pin#1 pop up there.
4. Do the same for the other point. Don't know how many times you can add a clipping point.
5. Then go to the Menu/Brush Clip/Flip Normal to change the direction of the clip. There is a shortcut key for that also.
Hope that helps.