Over arm chops are just chops which have been carried over the top of the receiving hand, rather than inside of it.:
This is a much larger movement than a normal chop, to exaggerate it still further, hold the club at the end of the handle. The club remains in the body plane throughout, and when the chop has been completed, the club should be horizontal. From this position, throw it up with an inside flat spin, to be caught by the other hand.
Over arm chops can be fitted into a cascade in the same way as standard chops. I have seen this trick done continuously from one side, but never on every throw.