A Twisty McMarx is a hoverboard trick. You'll need to do this after you've won the hoverboard race on Rilgar, as the hoverboard tricks are unlocked by winning it.
To do the trick itself, you'll need to either hold down left or up on the D-pad and then press R1, R2, L1 and L2 in any order. Be sure to hold it down for an entire flip, or you won't do a Twisty McMarx.
This can also be done at the Gadgetron HQ hoverboard coarse.