On a selfbow if you cut or shave through a growth ring on the back of the bow, you need to go to the next ring. The back of the bow needs to be on one growth ring for the entire length of the bow. It gets especially tricky when you have to deal with knots or very small rings. Growth rings about 1/8" or so are fine, but you'll find some with less then 1/16". The growth rings on a locust sure look nice, but I heard there are problems with heating/cooling the resigns.

The "back" of a bow which is the part that faces away from you, you want it to be one continuous growth ring. So you have to scrape thru the sapwood and then scrape down to a growth ring.

You can voilate to an extent this with hickory, hophornbeam.