Output stages of electronic circuits
How to exlain without diagrams? lets try (sorry if it's pathetic as a result)...
If an IC output can be imagined for a moment as a switch; this switch forces the output to ground working against the output's load (a light bulb who's other terminal is connected to the positive supply) it's easy to see how the 'switch' controls the 'bulb'.
Add a second switch in parallel with the first and either will light the bulb, but both have to be 'off' to extinguish it: A simple 'OR gate' - no problem.
Some ICs can have outputs connected together like this (open collector circuits for example).
IF the switch isn't a simple one -- it now has a change-over action with one side to ground and one to the positive supply; the moving contact being the output (the same load as before)
A single switch like this works exactly the same - on-off -
A second switch with the 'outputs' connected together - switch either on and BOOM
expensive smoke!
A dead short across the power supply.
Lots of ICs have outputs like this - op-amps, logic ICs, blah blah.
(Luckily some have protection or limiting on their outputs - 555 timers and 741 op amps but it still isn't good and can be unpredictable).
The diode 'OR' gate I suggested prevents the outputs from 'seeing' each other - it effectively converts 'change-over' into simple switches :wink: