This is a tutorial showing how to move from plank to upward dog. Chatturanga is a very difficult pose requiring a lot of abdominal and upper body strength. Holding plank is a way to increase that strength without putting the shoulders at risk. Proper alignment of upward dog is showed here as well.
You don't ever have to jump back or forward during your sun salutes, but if you do decide to jump back, please do so with your abdominals engaged and land in chatturanga with your elbows bent at 90 degrees.