This is the brand new Profile Design HC Bottle. The HC stands for 'Horizontal Cage.' After years of bucking the BTA trend, Profile Design has seen the light. With the HC bottle, they've clearly embraced the notion that consumers want these types of products, and they're satisfying that demand with a really smart offering that is going to wow a lot of people.
The bottle top works just like a standard bottle top, but it's nice and bullet-shaped. This probably doesn't much, since the thing sits between your arms anyway, but the bullet shape does help to fill that gap a bit better than a standard bottle top.