Take a look at your slipper setup, notice how the outer most plate has a square hole and is keyed to the shaft while the inner plate has a round hole, but then a square backing to lock it to the rear belt pulley.

What the stock setup does is allow the belt to slip independently from the rear, so while slipping the rear will always turn as long as the plates are moving since it's keyed to the shaft, but then the front pulley plate can slip to the point where no power is going to the front wheels especially during the first moments of acceleration when there is weight on the wheels. which is often the opposite of what you want to happen.

The pin mod locks that inner plate to the shaft just like the outer plate so that any time the shaft turns both front and rear will turn instead of letting the rear slip faster.

This is why people recommend the 19t up front with no pin mod, this will make the front wheels turn faster with the limited amount of power given by the slipper, where the pin mod accomplishes this by locking down front and rear.

Speaking of which, does anyone know what the front/rear bias actually is when using 20t for both pulleys and the pin mod? Does it make it true 50/50 (or 100/100 forget what the notation is) ?