Straight Ahead & Pose-to-Pose

Straight ahead is another animation principle. It is hand draw & the animator draws the first single frame and continues drawing every single frame until they get to the final frame. And in order to capture single frames the animator usually draws the in between frames with unusual movement. In other words, the animator draws the first and last frame and then they draw the in-between frames as well.
Pose-to-Pose is literally the animator planning their action pose to pose. The poses are the important frames of the story that best tell the story that the animator is trying to tell. This principle is great for planning for poses and timing.
*The picture illustrates straight ahead (top) & pose-to-pose (bottom).

Reference Link:

http://www.rosenfeldmedia.com/books/mobile-design/blog/animation_principle_four_strai/

http://www.evl.uic.edu/ralph/508S99/straight.html