I searched for the circuit online and found this website explaining, but his explanation does not make any sense to me. He says that when you activate T1 and then T4, this "energizes" the relay, thus activating the other circuit. This part makes sense, but the following has me confused. When you let off of the touch switch for T1 and T4, the light continues to stay on. By my understanding of relays and transistors, this doesn't make sense. The transistor is acting as a switch, providing ample current to the relay, but should it not then cut off when let off of the switch?
Secondly, T2 and T3 are activated to turn the relay off. This part makes a but more sense as the high current from the darlington transistor would draw all of the current from the relay, I believe.