Once you strap the ~$149 device to your forearm (that's where it's worn no matter what muscles you're focusing on), you'll sync it with your smartphone's dedicated app. When you're finished, it will beam the data it collected into easily digestible graphs, charts, and activity feeds on your phone. Lost count of your reps? It kept track. Wonder which rep you "peaked" on? It'll let you know. Not sure if you've rested long enough between sets? There's even a countdown timer.
It's also easy to send your trainer stats after each workout like a scorned lover, showing them how well you're doing without them.