What is it?
Magento module Webshopapps/Matrixrate allows you to give customer free shipping option if order totals will cost more than some money.
This module calculates only products price without included tax.
You have a store. You have product with base (real) price $8, but with tax (eg: $3) it will cost $11. You set in store settings that on the frontend part for customers must be displayed only price with included tax ($11). And you set in Webshopapps/Matrixrate config that free shipping option must be shown for order amount > $100.
Customer buys 10 products by $11 each, but module will calculate only base (real) price - $8 for each product. In that way, customer will see order total amount as $110 but he will have no free shipping option.