Its not the most sophisticated security system in the world. Its not even close to modern. But we have to admin that it is brilliant. And in some cases its more secure than a simple codelock.
A number of buttons in prallel (3 in this circuit) are used to pull the base of the load transistor directly to the ground. A number of buttons in series are used to pull the base HIGH through a resistor. So, if the operator pushes simultaneously all the series buttons, the base will be pulled HIGH and the transistor will be driven to saturation, powering therefore the load.
If ANY of the parallel buttons is pressed, the transistor will be driven to cut-off. The transistor cannot turn on in that case.