True, but it also acts as an anchor, often creating so much pressure it can break the attachment points, or if it does come free it will fire back at you even faster/harder.
The best leash solution is a reel leash from oceanus, its like a retractable dog leash only MUCH stronger. You can attach it to the end of your board, when you come off it just reels out, then slowly reels back in bringing your board back to you.
It almost acts as a shock absorber, totally stopping the slingshot scenario.
The other advantage is that the leash is straight from your harness to the board when you're riding, so there's nothing flapping around to get tangled round your fins/legs etc.
The only disadvantage I found was as you get better and can hold down more power, you are lower in the water and the leash can then drag out, acting like a brake!
But methinks you are a little away from that point......