Complex sentence: Many swap bodies are fitted with four up-folding legs under their frame, to make it possible to change or "swap" their body from one carriage to another, or to leave the swap body at a destination, without using a crane or hoist.
Split into simpler sentences: Many swap bodies are fitted with four up-folding legs under their frame. These legs make it possible to change or "swap" their body from one carriage to another, or to leave the swap body at a destination, without using extra equipment such as a crane or hoist.