A lot of phones have a method of opening the camera quickly when the device is asleep, but not all of them are reliable or all that easy. Snapshot aims to make camera access quicker by starting it up as soon as the phone wakes up, provided the device is in landscape orientation (i.e. the way you're supposed to be taking pictures). It doesn't matter how your phone wakes up—it just so long as it does.
The app's default behavior only launches the camera when you wake the device in traditional picture-taking stance. In portrait mode, Snapshot doesn't do anything. The app has a few configuration options that let you add a delay for opening the camera and (if you upgrade to pro) other orientations like upside down and face down. I'm not sure why you'd want to do that, but the 270 degree landscape might be of interest to you lefties. The app's free definition of landscape is the orientation where the bottom of the phone is on the right.
Snapshot runs a notification to keep itself active in memory, but there's an option to disable that in the settings. If your phone has a lot of RAM, it might be fine to disable it, no promises, though. Because it only polls the accelerometer when the device wakes up, it shouldn't have any detrimental effects on the battery, but I haven't been testing long enough to know for certain.