On the joystick side of the circuit, when the Extra Button [5] is pressed, it sends a signal to Button 2 [7] on the PC side. When Button [6] is pressed, a signal is sent to Button 1 [1]. The +5V on the joystick side goes to the two +5V on the PC end; these volts are used in order to maintain the transistors, resistors, and diodes between the more important components on the Joystick end (Forward [1], Backward [2], Left [3], Right [4]) and the PC end (+5V [1], Pot x [3], Pot y [6]). Ground on the Joystick side goes directly to Ground [4 and 5] on the PC side. Forward [1], Backward [2], Left [3], and Right [4] on the Joystick side go through the electrical components diodes, transistors, and resistors.
After going through these electrical components the signal is sent to +5V [1], Pot x [3], and Pot y [6] on the PC site. There are potentiometers as components in the actual circuits as well as on the PC site. The ones in the circuit can take between 0 and 150K ohms. Pot x and y are potentiometers on the PC side, which control the amount of power being used. This is used for joysticks because it is possible to move the stick forward but not all the way forward so that for example the object or person in the video game moves but does not go full speed ahead. For example in a race car video game, moving the stick half way forward would make the car go half as fast in a forward direction.
This relates to the game port where 7= but4, 6=but3, 5=but2, 4=but1, 3=stk4, 2=stk3, 1=stk2, 0=stk1. 3, 2, 1, and 0 are all sticks therefore they each have a hexadecimal value and range from 0- 255. For example, 255 would be full speed forward. This whole game port and potentiometers are on the side of the PC, which receives the signal from the joystick and makes the activities in the game work. Also on the side of the PC, there are buttons like I have stated above like in the game port. 7, 6, 5, and 4 are all buttons and that means that they can only be 0 or 1. 0 means low and nothing happening and 1 is high with things happening. However the state can only be at 1 during the time the button is pressed so therefore when the pressure is released off the button, it goes back to 0, the original state.
Going back to the circuit and the electrical components diode, transistor, and resistor. These do not play a big role in the processing of the signal to the PC, they act more to limit the current and protect the PC circuiting just like the +5V does. The diode only allows the current to flow in one direction therefore not allowing the current back to the joystick side, which would cause a problem if the current were allowed to flow back. So this allows the current to only go from the Joystick to the PC. The transistor in the diagram is a PNP BJT Transistor, which if there is voltage in the base, will pass current from the collector to the emitter. The resistor limits the flow of current when voltage is applied. Therefore these components are more for limiting the amount of current and protecting the PC circuiting.
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