You show four large transparent plastic envelopes. One contains a some cash!, while the other three have a piece of white paper inside. On the back of each transparent envelope there is a colored dot (red, yellow, blue and green). You now show four jumbo cards numbered 1 to 4.
These too have colored dots on the backs. Three spectators freely choose a Jumbo card and receive the envelope with the corresponding color. Naturally all three spectators end up with the envelope containing the white piece of paper while you can keep the cash.
This effect uses only the four cards and four envelopes. No switch is needed. The spectators can freely choose any of the cards. No force involved! At the end everything can be examined.
Jumbo Mental Jackpot is the parlor/stage version of the Mental Jackpot Pocket trick but employs a different method!