In this trick the devilstick is spun horizontally like the main rotor blades of a helicopter.
Start from Single sticking, lift with a little more force than usual so that at the point of each catch the devilstick is resting horizontally on each handstick rather than at an angle. To force the devilstick into a flat spin you need to alter the way you make each throw, one hand needs to push the top of the stick away from you the other needs to pull it in towards you. Decide which hand will do the pushing & which one will do the pulling. Both hands need to work together to get this trick working. If you push more than you pull you will simply end up chasing after your devilstick.
The pulling hand
Keep catching the devilstick as normal but make sure that the devilstick lands close to the tip of your handstick. Throw the devilstick by tilting your wrist back so that the handstick moves from being held flat to the ground to pointing upwards. Keep the handstick in contact with the devilstick for as long as possible. The top half of the devilstick should roll down the angled handstick towards you.
The pushing hand
Do the opposite with the other hand to push the devilstick away from you. Catch as normal but instead of catching near the tip, aim to catch the devilstick as close to your hand as possible. To throw lift your arm up but tilt your hand so that the tip of the handstick points downwards so that the devilstick rolls down & away from you. Again try to keep the handstick & the devilstick in contact with each other for as long as possible.
The devilstick won't instantly go into a flat spin it will take a few throws for you to build up the momentum. As the devilstick starts to spin faster start catching the devilstick a little closer to the central waistband. Be careful because as the devilstick starts to spin faster it will also spin flatter, which means the bottom half of the devilstick may start catching on your knuckles.
To instantly go into a flat spin
Slow is boring. To look like you really know what you are doing you want to be able to change the plane that your devilstick is spinning in instantly.
Again start from single sticking, making sure that at the point that you make a catch the devilstick is horizontal. After making a throw from the pulling hand lift your elbow up & drape your pulling arm over the top of the devilstick so that the handstick is pointing straight down. The bottom half of the devilstick should swing up in between your body & your pulling handstick. Wait until the whole devilstick is lying horizontally then make a pushing throw with the pushing handstick as normal & at the same time push the bottom half of the devilstick straight towards you (this is basically the same as Double sticking but in a horizontal rather than vertical plane). Immediately drop your elbow to make the catch with the pulling hand & continue.
Oh, before I forget - learn to spin a helicopter both ways.