SO WHAT IS A GNARLY SPLIT?

U have to know what a gnarly split is cuz otherwise you wouldnt be looking to know how to use it, but incase u happen to not know, a gnarly is when u split one piece into another one of your pieces and make them rejoin upon contact, creating an extra large piece to enable you to eat larger opponents than usual.

HOW DO U GNARLY SPLIT?

In order to gnarly split, the following requirements must be met:

1) You are in 2 pieces, one larger than the other
2) You are ready to rejoin (but dont rejoin)

Once you are in this position, you should have two pieces slightly overlapping, ready to rejoin and one slightly bigger. When floating towards your target, try keep the larger piece behind the smaller piece.

IF U USE A COMPUTER USE THESE STEPS

Press ctrl, this should stop your pieces from moving, except the bigger piece behind should still be slowly sliding towards your small piece. You must not let it reach the small piece or they will rejoin. Aim your larger piece towards the smaller piece with the cursor and split.
(You must be holding CTRL the whole time)
The split piece will fly forwards and combine into the front piece, sending them both forwards.

IF U R USING A PHONE OR TABLET USE THESE STEPS

Lift your finger off the screen, the two pieces should stand still, except the larger piece will float towards the smaller piece, before they can rejoin, double tap quickly to split the larger piece towards the smaller piece and as the split piece hits the stationary one, they should combine and fly forwards.

It is recommended you practice a few times to get used to the timing before doing it in a real game, because if you stuff up, both pieces split and usually into a larger enemy, resulting in an instantaneous "game over".

I hope this guide helps, if anything is left out, please tell me in the comments so i can put it up. :)