Proper starts are vital to a good race. Many races are won or lost based on the start. Many coaches and swimmers neglect to devote enough time to developing efficient starts.
There are 2 types of starting block starts, the Grab and the Track. They are used for freestyle, breaststroke, and butterfly.
The Grab start: This is used for freestyle, breaststroke, and butterfly. The swimmer is on the blocks, toes curled around the edge, with feet shoulder width apart. Then grabs the front of the block. Knees are bent about 90 degrees ready to explode off the block. The weight should be on the ball of the feet, not on the back of the heels. The swimmer should be looking at their destination so when the gun goes off they explode off the block.
The Track start: This is also used for free, breast, and fly. The difference is that the swimmer has one foot behind the other foot. Some swimmers find this easier, it's usually a matter of personal choice. The swimmer can still grab the block or simply let the arms hang, if they lack the flexibility to grab.
For backstroke the swimmer starts in the water. Feet should be placed about 7 inches apart with only the toes just under the water or just out of the water - whatever is comfortable and whatever will avoid slipping. The head should be pulled towards the block, knees should be at 90 degrees (this is the angle which the legs have the most explosive power). At take-off the hands should be pushed away first and the head thrown back. The legs follow with as much explosive power as possible. In the air the arms need to go above the head - over the top might be faster. The back should be arched so the swimmers enter the water hands, head, upper body, then the legs. Ideally the swimmer should be about 1 foot under the water. Start to dolphin kick until you are about to surface. The first arm pull is underwater and the second one is out of the water.
Be careful of unnecessary movements once the gun goes off. Many new swimmers, upon hearing the gun, will re-balance themselves, or perform wasted moves. All these things will delay a quick start and swimmers need to remove all unnecessary movements.
Developing efficient starts is not something to be neglected. Start early and practice regularly. Have the swimmers practice their starts at the start of the workout before they get tired so they have the energy to put the effort into the starts. Organize little competitions, e.g. who can dive and glide the furthest, or who can dive over a floating object in the water.