Imagine you are floating in outer space. There are many rocks floating around you, though everything is generally motionless… just sort of spinning around suspended. Now, if you take and throw one of these rocks, it will push you backwards with the same force with which you threw it, causing you to move the opposite direction through space.
The only way to slow down is to hit other rocks with your body, which will absorb the kinetic energy.
Every volitional action you emmit is throwing a rock. If you throw a rock upwards (positive), it will push you downwards and give your feet somthing to balance on for a moment. If you throw a rock downwards (negetive), it will thrust you upwards and you will hit your head on a rock.
As such, karma is punching a thorny shrub.