Here is something I learned from watching knappers in TN. I call it hang nail because I don't know the name of the technique. As you can see from the picture you will be striking the platfrom but with a guide. OK here is the deal if you have learned what a ridge is and you know how to set up a platform the next step is to hit the platform is jut the right place. Well this technique will help you do just that. See if you hit the platfrom above center the biface will receive to much shock and the piece will break. It will break also if you hit at the wrong angle. Now a couple things need to be done before using this technique. So first made a good platform to strike. Second find a ridge on the bottom side that you want to remove. Third make sure that the platform is abrade. You might want to isolate the platform if needed.
Now that all of the above has been done you need to setup your left hand. Hold the preform in your left hand, put your middle finger on the ridge, it will be just below your thumb. Place your thumb in the location that will be the center of where you think the bopper will land.
Here is the cool part - using your thumb nail place it down the platform to about middle. Now you can't hit the platform too high. You can't break the preform unless you drive the bopper through your thumb. I have been holding the bopper at about a 90 degree angle. If you do hit high your thumb will act as a ramp to guide the bopper to the platform. That was easy!
