Here's one of many studies done in the last few years on a very popular subject:
TRACK START VS. GRAB START: EVIDENCE OF THE SYDNEY OLYMPIC GAMES
~Vladimir B. Issurin and Oleg Verbitsky, Elite Sport Department at the Wingate Institute for Physical Education and Sport, Israel
"Two concurrent swimming start techniques - Track start (TS) and Grab start (GS), are presently particularly popular among swimmers and researchers. Despite the relatively large amount of publications, the outcome as to which is remains inconclusive."
Note: If in your own personal swimming experience, you found one technique better than another, tell Dynoswim which is better and why.
I think it is a matter of personal preference. I teach my kids both and have them use the one that works best/is fastest for them. I use a track start just because I was less likely to false start.
I was taught the grab start 20 years ago and continue to use it today.
I like the grab start because I have more leverage.
The grab start, when done in earnest, allows the swimmer to hit the water at several MPH more than the track start. If the ensuing streamline is held, hitting the water faster will allow a quicker time to the 15 meter mark. Also, with the grab start, the swimmer should be able to cover more distance in the air obviating more water travel...moving through air is faster than moving through water.
I like Ed's perspective.
Personally I use a Grab start, that is what I was taught.
Professionally I don't understand why you would use a Track start, unless the blocks are at an angle that you can push off with both legs at equal pressure. In Track and Field they use starting blocks.
From experience I feel so many kids use a track start as a cop out as they lack the flexibility to get down for a grab start.
And they only push off with the front foot and have legs apart for a horrible streamline.
So I agree with Ed's take