I used the triangle when Iworked at a grocery store bakery and I FINALLY figured out how to use it. They never really trained me, so I figured stuff out on my own.

I held the triangle at about a 45 degree angle on the cake side, and went around with gentle pressure. I went around once or twice, depending on how well the icing was stuck to and smoothed out on the cake.
If I was have trouble getting it to smooth, sometimes I would go backwards from the direction I iced it in (I used the icing tip to ice it) and that would help.
Another trick was to start at a corner, if icing a square or rectangular cake, and then go back to the same corner and scrape in the opposite direction to help get more squared corners.
When you are done, gradually remove the triangle while still moving along, so that there is no ugly removal line. I always put that side in the back, too.

Hope some of that helped. As I stated, they never trained me, I just figured some stuff out that was helpful to me and wanted to pass it along.

i absolultly loooove my cake triangle!!! would die without it!!!