The manual seems clear to me:
Loading screen
Use:
Double click to create new page with this feature. Edit page.This tweak allows you to have an alternative design that will be shown to user while the main page is loading.
When you create new page with this template, you’ll see there is a “Load screen” layer. This layer contains the alternative design. When page is loaded bu browser, only this layer is visible. When page is loaded, this layer is hidden and the design is revealed to user.
Edit the content of this layer any way you wish. But do not change layer order
Basically, you start your site with the LS Tweak, it creates the page with the correct layers and layer order.
You create your Loading screen content on the !Load screen layer and your normal page content on the MouseOff layer.

If you need a longer page than the LS tweak creates, drag the bottom edge