Concealing upgraded functionality in an apparently lower-level product, which is then restored by a ridiculously simple action, such as opening up a panel and turning a screw.
In the heyday of the mainframe, hardware upgrades were installed ahead of time and deactivated. When the customer was finally up-sold on the improvements, a sales engineer simply crawled inside and did little more than turn a screw.
The reward/effort ratio was so high that this practice was called the Golden Screwdriver.

Certain software programs appear to be sold in different versions, but in reality, these functionality tiers are controlled by the serial number the user is given to enter during registration.
by desserteagle October 31, 2009
When some loses the screwdriver from their set, and their tool set now looks incomplete.
Guy1: "dude your missing a tool".
Guy2: "Yeah it was a screwdriver, THE GOLDEN SCREWDRIVER!".
by -robstar February 3, 2018