If someone jumps the gun, it means they start too soon, before the starting pistol.

jump the gun是指有人在发令枪之前过早地开始行动。

It's the starting pistol for the campaign.

这是竞选开始的信号。

After the starting pistol is fired, the boat races in dragon shaped canoes glide through the water accompanied by the beating of drums.

发令枪响后，龙形独木舟伴随着鼓声滑入水中。