Calculated Risk is a completely impromptu effect in which you are seen to unlock two borrowed iPhones.
Using nothing more than two borrowed iPhones you will be set to amaze your spectators with your deductive abilities.

Nothing is written down or spoken out loud.
No special apps required
Almost completely self working
The core principle involved opens itself to many routine possibilities.
Providing that you have four fingers and a thumb then you will be able to perform the effect.