Gesture sketching is one of the most important principles of life drawing – which I’ve talked about before in similar posts. The importance of this method is in capturing the shapes and their elements especially creatures (which usually don’t sit still a lot) and to do it quickly in a few seconds (e.g. the first sketch below was done in about two seconds). With practicing you’ll be able to store the shape you’re sketching in a faster intuition, and in many times continue the sketch from your memory.
Another advantages of this technique is to train your (mind) before your hand on sketching quickly, and perhaps that’s the most important thing for drawing for animation (as the pure classical animation is created usually in 12 – 24 f/s and demands a high promptitude).
Note: using a technical pen is much preferred, to draw with sliding, smooth & clear lines. And I think that the most important reason for using the technical pen is not to be focusing on your mistakes, so you go back to edit and clean – as with the pencil. And yeah!: Get a BIG sketchbook/s ^^ I use Winsor & Newton, it’s is a good kind and economical.