Jerk Dip - Smooth Start
What we don't want to do is begin so abruptly that we actually drop out from under the bar and create separation. Any disconnection between the bar and the body during the dip means you're not going to be able to get as much acceleration on it and you're going to tend to push it forward.
Two main things that you need to work on to make that happen:
First is ensuring that the bar is racked correctly and totally secure in that position. In other words, your grip is not overly tight, your arms are not pushing up into the bar and creating an initial bit of separation that then just gets amplified as you dip. Let that thing settle in all the way against your body.
The second part is making sure that you have tension throughout your quads and glutes without having your knee in hyperextension. If you stand in that passive hyperextension position with relatively loose quads, there's slack in the system.
So when you begin, there's actually a moment where you are basically free falling before your muscles catch up and are able to control you.
Get tension throughout the quads with the legs straight. I'm not saying bend your knees to start, straight leg, not hyperextended, and initiate smoothly, and then you can go as fast as you need to go.
Optimal dip speed is a function of your natural physical traits—more explosive = quicker/shallower dip, and less explosive = slower/deeper dip.
But in any case, the initiation of the dip should be smooth enough to maintain connection of the bar and body rather than so abrupt that it creates separation.