They have explained previously in this thread some of the usage which would be in contravention. This includes bypassing shaping, sharing the connection (as in an open wireless router, not as in sharing with a member of your household of course) and downloading at line speed 24/7/365. It is uncapped because your connection will not be terminated after you have downloaded a predetermined amount. It will be terminated if you contravene the AUP/T&C.
One of the points which is not clear is the issue of constant downloading 24/7. HTTP downloads generally come down at line speed, so if you queue up GBs and GBs of HTTP downloads, then you can max out your line for an entire month. I suspect that this would be considered contravention of the AUP. If you queue up 8GB of Steam downloads over a weekend, that's fine of course. They look at usage patterns over time, not specific "bursts" of downloading or total usage. You have to make an effort to contravene the AUP -- you must intentionally, repeatedly and continuously bypass the shaper or queue up downloads such that they run 24/7/365 and are only paused while you swap in another hard drive.