What is an adaptive defrost in newer models, which replaces the defrost timer?

one of the functions of the Jazz Board

Defrost Operation:

The Control Board adapts the compressor run time between defrosts to achieve optimum defrost intervals

Enter the Forced Defrost Mode by performing the following sequence of events:

1. Hold the refrigerator door light switch closed.

2. Press the Refrigerator Temperature Down keypad 3 times consecutively within 10 seconds.

Source: Maytag MFF2558KRW french doors, bottom freezer