There are many steps to an effective backstroke start, but two seem to be the most neglected by the learner, and the lack of these two items is a significant slow down in your 50 yard back time.
Get your head back. As you begin to drive with the legs you need to throw the head back to ensure a back-dive like entry into the water. See the head position of the swimmer. You want to enter on your head and have your body follow through a hole, not land on your back causing you to lose your forward momentum. To do that you must get your head back early off the start.Once you are in the water, you need to check your downward momentum by using a quick dolphin kick to change the direction from towards the bottom to towards the opposite wall.
Next, Get your hips up. The successful start gets airborne. If you do not throw your hips up, you will sit down in the water and your start will not go far. You can see in figure 2 that swimmer has successfully thrown their hips high to become airborne.
All things being equal, successfully adding these two items to your start will take up to 5 seconds off your 50-yard backstroke time.