The word “lead it” is probably not the best word choice, the “lead it” is just a way to tell you that you are on your way to the place from which multiple laps will start.
Al races start at the start banner.
but Zwift have to somehow get you to the “start position” or the selected route. Like if you want to race laps around the volcano then you have to get there so there is a short “lead it” or “bonus road” or “lets get there” portion.
This could actually be a lot of fun, but at what speed does the fence move, because it is w/kg and what is the weight of the fence. Two riders with different weight will travel at different speed on a flat road if they hold the same w/kg.