The following video demonstrates v-ups:
A V-up begins laying flat on the floor with your arms extended over your head. Then, you’ll simultaneously raise your legs and torso using your abs until your body makes a V when viewed from the side. If this movement is too difficult, you can substitute leg raises, in which your head and back stay on the floor, and only your legs come up. Place your hands palms down under butt to help with balance for this latter movement.