Kenny, I think you're understanding it okay in regards to a stave. If you're chasing a certain grain that you want to be the back of your bow and you accidentally pulled up to much wood with your drawknife causing you to get into the next lower grain, then you've violated the grain you were chasing. Not so bad when chasing a grain and you can make the decision then to chase the next lower grain. Much worse when you have a roughed in bow and you violate or somehow work through the grain that is the back of your bow.
I've never made a board bow, but from what I've read and hopefully understand it is the same scenario for the most part. You should look for a board where the grains are all running parallel with each other and the edge of the board throughout the length of the intended bow. There shouldn't be any grain line that comes to a point or ends within the bow length...except you might get away with it if it's in a nonbending area such as the handle.