It's the second pedal push after the traffic lights. The first foot is on and you're moving.
Then the other foot begins to feel its way. It gets a tiny, insufficient piece of pedal and plummets earthward.
If you're in the saddle, you might get painful pedal strike around the ankle region.
If you're out of the saddle, your crotch hits the saddle hard and you're lucky to walk away with your junk.
If you were up out of the saddle - really trying to put in power - you can end up missing the saddle with all but the very back of your butt, and having the nose of the saddle wedge right in your crack.
BONUS TIP: Of all the cycling injuries, saddle-induced crack damage is the least discussed and most stigmatised, but really, who hasn't known its torment?
I said: Who hasn't known its torment?