Although lightweight, the Arch EX 650b (aka 27.5") rim can handle aggressive trail riding. The internal arch design provides durability and stiffness, two assets especially important for heavier riders or any riders known to be hard on wheels. It is quite possibly the best rim for hard cross country or all mountain use.
Stan's rims employ bead socket technology. BST marks a huge step forward in tubeless technology, allowing for lighter rims, lower sidewall heights, improved durability, increased traction, and decreased chances of pinch flatting. BST rims allow the bead to properly lock into the rim for the best quality airtight seal. The bead/rim interaction is superior even with an inner tube.
The low-profile shape of BST rims makes them lighter for less rotational mass for weight saving where it matters most. BST allows the tire to expand to full size so you can run lower pressure with less pinch flats and your tire functions more efficiently for better cornering, traction and reduced rolling resistance.