Theft starts automatically as the phone is turned on, and then always keeps detecting whether the phone is turned off in a wrong way, or your SIM Card is changed. It also keeps checking messages arriving for an activation text you previously defined (See the "SMS text to attack" in Program Settings section). Then ?Theft’ will attack the thief in three ways:

With a permanent text on the screen saying that THIS MOBILE IS LOST OR STOLEN along with any other text you provide; such as your name and contact telephone number.
The phone will send messages to a list of recipients you created for ?Theft’ to inform you that the phone is locked (It will also inform you of the new SIM number if someone has changed your SIM Card.).
A very loud long siren will sound from the phone alerting people in the vicinity and disturbing the thief