Note : I am left-handed, so you will reverse the directions if you are a righty.
The Technique :
Hold the deck firmly with your weak hand (right side shown for me) with the deck firmly and horizontal to the floor.
Pinch the deck with the strong hand with the thumb and index finger. It’s the same grip you use to use a remote control.
Pull your strong hand (pinch side) towards your chest. The index finger splits the deck. You must pinch hard!
Continue to pull hand towards your chest, turn your weak hand towards the floor. The deck will rip – not twist – if you have a tight grip. The tear line will be clean.
Continue to pinch the deck hard and pull back towards your chest until you separate the deck. The line will be a clean cut right through the deck. With this method you can put the cut anywhere you want on the deck.
It takes pain tolerance and hand strength with this method. Keep practicing. THERE IS NO TRICK. Rub some lip balm on your hands after you tear if the skin is raw.
I can now rip decks of bicycles in less than 3 seconds, and quarter them in a second with this grip. It makes you strong.