When you are idle your brain tries to explore, in one such exploration I adventured this feature and tested the same in my Samsung c160 mobile.
The gist of the feature is that you can store the sms receivable number of the ones who care for you (upto five). when you are in emergency (eg situations that stroke my mind are I am disabled to talk, diseased, amidst an accident, robbery, killing spree, hostages, hijacking etc), you need to press the cancel button four times continuously, predefined sms will be sent to your care takers, and when they call you, you mobile switches on automatically without any ring and they will be able to hear the happenings at your side (imagine with 3G they will be able to view too) - then you can extend the story how you could be saved.
The testing I made is here
path Menu > Messages >SOS messages > Sending options
Sending Options - enable 'on'
Recipients - type the mobile numbers of your caretakers
Repeat - choose how many times the sms has to be repeatedly sent
Message - view the message that will be sent
Help - instruction how to use this feature
I locked my mobile, pressed the "c" button four time consecutively, the sms is sent to my wife's mobile (which I stored in recipients) and I called back from her mobile, which is connected without any ring or need to answer. Then I pressed the end button to cancel the call and come out of this SOS.
Help says "when key lock is active this will send the SOS message to your selected recipients. If they phone you back their call will be automatically connected. After sending the SOS message all other functionality on the phone will remain unavailable until the end (red) key is pressed."
Have you seen the tamil movie "Pokkiri" starred by Vijay and directed by Prabhu Deva, i recall a scene in that when Asin and her brother were surrounded by rowdys the boy will call Vijay's number and talk to the miscreants telling them something similar to "wait guys you will see our power as soon as the train reaches Nungambakkam in 5 minutes", which is heard by Vijay at the other end and he rushes to their rescue - cinematic but realistic too.