I don't know if it could work on upright machines, I suspect not with your hand since they work differently and use gravity to drop coins from the spinny thing. The escalator would push forward on the coins even if you pushed them back they would jam up often(I would always cram my cup in there to capture coins and it would jam up easy). I think most slant tops would be sesptibale to this. I think the key was also that there was a long travel shute the coins traveled on so you manipulate the coins from right there where you put your hand to get coins. The tray shute is significantly smaller on a bar top than an upright machine, perhaps its only a bit bigger than 7" x 7". Even if it was technically possable on an upright, it would look obvious since you would have to have your hand and arm up the machine. I'm not even sure if it would fit up there or you could reach what was needed to achive this. On a bar machine, I dont even think you would have to position yourself to really block anything. Your hand and arm would block what your fingers were doing. It was natural for someone to have their hand in the tray shute to caprure their coins as they were being spat out. To look absolutely natural, I think you would have to go back and forth to your cup with coins and that might mess up your hand positioning and have to readjust before your credits were at zero. There would be no good reason for someone to have their hand in the tray receiver just sitting there and not pulling out coins. I never heard of anyone getting busted for this and I never saw anyone doing it. I would definitely notice if someone was keeping their hand in the tray longer than necessary.