What does 'bidirectional' actually mean on a scan tool?

A bidirectional (or 'active test') scan tool can send commands to vehicle modules — cycling an ABS pump to bleed brakes, commanding a cooling fan on, running an injector buzz test, resetting an EPB to swap rear pads — not just read codes. Across the mechanic-community threads we read, this is the line between a $50 code reader and a tool a working tech will actually use.