Use a really sharp knife, plus some elbow grease, to cut corn lengthwise. Some pieces are very easy to cut and some require more force. The safest way to cut corn lengthwise is to put a flat edge on a surface and then cut. Do not put lay the corn on the kernels and try to cut that way. There's a chance that the corn will roll and that's very dangerous. That's how Pete does it when he cuts the corn for corn riblets. If I'm cutting the corn, I find it easier to cut the corn in half first (which basically contradicts the last sentence because you have to lay it on the kernels to cut it in half). But it's easier to cut half the corn into two cob like pieces than cutting the whole corn lengthwise.