This notch forms the center vein of the leaf. No matter what sort of icing you're piping leaves with (royal or buttercream), you'll want the consistency thin enough so the sides of the leaf on either side of the notch will fuse together as you pull the tip away. I just can't bring myself to show you what the end result looks like with icing that's too thick; I don't do ugly on this blog. Believe me, you'll know a do-over leaf when you see one. If your icing isn't smooth and creamy, add a dab of piping gel. Start small; a half-teaspoon of piping gel for half a cup of icing, or thereabouts. The icing I used for these leaves (Wilton's Ready-to-Use Decorator Icing) started so thick I actually gave it a few seconds in the microwave just so I could get it malleable enough to stir in the piping gel. Using an Ateco tip #69, I piped a few of the most basic leaves. They're all made by holding the tip parallel to the surface, more or less. Start by squeezing firmly, then decreasing pressure as you draw the tip away from the base to form this first leaf:
shell border; move the tip slightly away from you, then boomerang it back over itself. Pull the tip away so each ruffle ends in a small tail. Repeat, repeat, repeat.
This side view shows you about the distance between each ruffle.
Here's a ridiculously thick ruffle, made using the same sawing motion as the second ruffled leaf above; just done to the extreme. This border would please folks who like a little cake with their icing. (P.S. I've seen this method used for piping pot leaves; just stop sawing back-and-forth when the leaf is the length you want and pull the tip away to form the pointed end…)
Don't forget; you can also use leaf tips to pipe flowers, like the tiger lily; the full instructions are here.
One other tip you can use for piping leaves is the Wilton tip #352, which is basically all notch. To form the center vein…