You show a virtual cube on your phone, which can be mixed with finger swipes. You also show a real cube. Then you try to make a prediction on the phone, while the spectator is mixing the real cube. You put your prediction (the phone) face down, and wait for the spectator to finish mixing the cube. When they are ready, you turn over the phone, and match the virtual cube to the real cube - and they are identical!
Select "Virtual cube match" from the routine list. There is no need to select a secret screen type, this routine will always use the virtual cube screen. There is no need to set up anything else. You just have to connect a bluetooth cube, and you're ready to go.
After starting the routine, you will see a fully solved virtual cube on the screen. You can show the spectator how you can mix the cube using finger swipes on the screen: just swipe the side you want to turn in the desired direction. You can also flip the cube up/down by swiping one finger up/down over the blank space under the virtual cube. You can now ask them to mix the real cube, and they will see that there is no connection between the real and the virtual cube (no need to emphasize this though, it would be suspicious).
Then you say you will make a prediction by mixing the virtual cube, while they can mix the real cube. You don't show them the screen while you are mixing. When you are ready, just place the phone face down on the table.
While your phone being face down, the virtual cube will clone the real cube, no matter how they mix it. When the spectator is ready mixing, they should put the cube down. Then you can turn over the phone to show them your prediction, which will match the real cube.
If they mix the real cube some more, the virtual cube won't change anymore (even if you turn the phone face down again).