It uses ajax and jquery and asks the server for available stock and replaces the 'Add to cart' button with some configurable HTML.
It should be mostly Plug&Play except for the configuration of the replacement text.
Dislpay attributes type supported for AJAX validation:
Select box
Radio buttons
It also provides server-side validation so this module can prevent the addition of out of stock items even if JS is disabled or if the user clicks on an "Add to cart" button too quickly.
Validations of stock are performed on the following places by throwing error messages accordingly:
Single product add to cart forms (teaser, full and catalog view): If an item out of stock or, if the item you want to add is already on your cart but you have reached the stock limit
Shopping cart form: If you try to add more items that the ones in stock of any product of your shopping cart
Order checkout and order review form: If the stock information has changed while the user is browsing the site (i.e. same item has just been bought by another user)
Dislpay attributes type NOT supported - they don't add variations to SKU, so not related to stock control: