Sorry I'll add one more thing. As in the photo above, gripping the inside of your front ski can help just set you in a straight line. Ie. Stops you over rotating and keeps you facing forwards, assuming you don't want to come off the rail switch. Its really good to practise sliding rails and boxes like this rather than just with flat bases as its a lot more controlled. On a box just use the left most 20% of the box and imagine the rest of it isn't even there (that's if you slide left foot first, obviously swap for right foot first).