Complex sentence: The propellers of a DC-6 are designed in such a way that, upon touching down, they will provide reverse thrust, in that the propellers will begin producing thrust in the opposite direction, essentially propelling the aircraft backwards, to slow it down.
Split into simpler sentences: The propellers of a DC-6 are designed in such a way that, upon touching down, they will provide reverse thrust. When this feature is activated, the propellers begin producing thrust in the opposite direction, propelling the aircraft backwards to slow it down.