Are you asking a generic question such as 'What are some of the techniques used to protect a UPS from short circuits'. Or do you have a specific UPS in mind (if so, the manufacturer might be able to help). Or is do you have in mind something such as 'How do you protect a dc power supply from short circuits?' Overall we (the audience) would probably like a lot more specifics in what you want. And, as a note and comment, this can take a lot more time to explain!!
Power supplies have a control loop that looks at the output voltage. If the voltage is not right then the supply changes something to increase/decrease the voltage to make it right. Something = duty cycle, phase, frequency, on time, ..... many different ways and many different type of supplies. (voltage control loop responds slowly)
Most modern power supplies also have a over current control loop that responds fast. If (and only if) the current is too high than "something" is pushed down very fast to reduce the (voltage, current, power).