Normal tethering via Desktop Manager will send the traffic out on a specific IP port designated by RIM for tethering. This makes it easy for carriers to monito this port and to either block tethered traffic or to charge you for using it.
Tether on the other hand sends, as I said, all its traffic out on the same port as the internal browser thus "fooling" the carrier into think youare just surfing from the phone. Now if you start downloading movies using Tether even a carrier will wake up and take notice. They will (not that word) detect the sudden increase in traffic.
The key then is to be sensible when tethering with Tether and let sleeping dogs lie.
Other products may do this too but I am not sure which.
