The current gain of the transistor can be used to extend the discharge time of the capacitor. The circuit has a 100nF electrolytic capacitor which serves as a storage capacity. It charges quickly when you press a PUSH button and when the key is released the capacitor supplies current to the base of the transistor. The LED will light up and dim slowly. High resistance base resistor gives a time constant of about 10 seconds. After that interval, the base current is not strong enough to excite the transistor for complete conductivity.
