In much the same way that boat boosting works, cart boosting allows the runner to speed up the cart a little bit per jump without losing momentum to anything other than friction. However cart boosting is particularly more difficult, as a cart is a flat surface about half the width of the boat. In order to make this work, you have to run to the right edge of the cart just before running off, and then reverse momentum jump and quickly change direction again back towards the right. It also has to be timed well enough to make sure you reach the ladder to hop over the saw.
Saves up to 2 seconds.