A necessary principle of animation, but my least favourite to explain. Straight ahead is frame after frame being drawn, no gaps so outcome ideally is more fluid. In 2D, straight ahead can distort the proportions of the character whereas with pose to pose the animator is missing frames for example they may draw the 1st, 6th and 12th frame and fill in the gaps. Pose to pose allows less distortion of proportions as the animator can see where they are aiming to get to. Combing the two allows the animator the best of both.