Chops is a rapid, very visual trick in which the clubs, instead of being thrown from one side to the other, are carried quickly across your body in downward 'chopping' motions.
As mentioned earlier a chop is more or less the same as an under the arm throw, except the emphasis here is on the club that is being held rather than the one being thrown. Try a few under the arm throws, just single ones for now, and watch the club that stays in your hand, i.e. the club in your right hand when you throw under your right arm. You may find that you use a little up and down motion to get your arm out of the way as the club goes under it, if you don't do this already then try to make it happen, it should feel quite natural and may help your under arm throws:
In order to change an under arm into a chop you have to exaggerate this arcing movement so that it is higher and broader, carrying the club over to the opposite side of your body. Since this movement is larger than the previous one it also takes longer to complete and so, to gain some time, try to catch the club-to-be-chopped higher than usual to eliminate the need to go up before you go down.
To start the chop bend your arm at the elbow and point your forearm straight up, now quickly straighten out your arm and at the same time carry the club down and across at an angle of about 45 degrees to your body.
Now do a small throw with one spin so that the club goes straight up. and catch it in the other hand. When you release the club your arm is not totally straight but still bends a little at the elbow.
Try all this for a while with just one club: chop, throw, catch, chop, throw, catch . . . . , your chopping action should feel comfortable and smooth. Now try to fit a single chop into a cascade pattern. If the whole thing seems too quick then remember to slow down your throws rather than the chops themselves - make the throws high and slow or use doubles. For quick chops make the throws with your wrist, for slow ones use a larger arm movement.
Continuous chops is usually done with single spins, however using doubles slows things down allowing you to fit in flourishes or other tricks. A word of warning about flourished chops: the speed of the chop can sometimes cause the club to fly out of your hand in mid-flourish. I managed to hit myself very hard on the head three times running when trying this for the first time. It still looked good though.