The frame twist test is a rather punishing test typically carried out on trucks, which because of their ladder frame-type chassis, is susceptible to bending when it is subjected to uneven load. Typically, test involves driving the truck over a series of ramps, staggered on either sides of the truck. A weak chassis will start bending as soon as one of the axles are not on even ground. As a result of the bent frame, you won’t be able to open the doors or the tailgate.
In the real world, it would mean difficulty in getting in and out of the vehicle if the vehicle is heavily loaded, and parked on uneven off-road ground.
The self-explanatory video produced by Mitsubishi Motors Malaysia (MMM) shown below takes the frame twist test one step further. Typically, a frame twist test is done by driving an unloaded truck over the staggered ramps, as shown in the subsequent video. Nevermind about what they are doing in the US, the folks at MMM would like to show how the Triton fares in a frame twist test, with over 1,000 kg loaded on its deck.