From Topo:

Technical trails have met their match with MTN Racer 3. This new update features an increased 33 x 28 mm platform for even more cushioning and protection underfoot, with improved toe sweep for a fast transition and smooth ride. The midsole gets an upgrade with our new ZipFoam™ formula for the perfect balance between soft cushioning and snappy response, while the Vibram® Megagrip outsole you know and love is back to deliver serious traction and grip when the trails get gritty.