I think he meant ,turn the key ,and as soon as it fires up you turn it off.
Just to many people , have some one else back up there trailer in the water while they are in there boat to find out they did not hook up the battery or even turn on the gas. The list goes on and on. Like where is the key !
Been doing it for over 15 years. Just makes the line up go faster.
What he did not mean was run it untill it was your turn to launch.