It's a verification system in the MIDI protocol used to confirm that the connection between two devices is still active.

Think of Active Sensing as a "life signal" or a heartbeat that one MIDI device (like a keyboard) constantly sends to the other (like a sound module).

The transmitting device sends short, frequent messages. As long as the receiving device is receiving these "heartbeats," it knows the connection is working.

If, for some reason, these messages stop arriving (for example, if the MIDI cable is disconnected), the receiver assumes there has been a failure.

To prevent notes from getting "stuck" and ringing indefinitely, it automatically turns off all active notes. It's important to note that not all MIDI equipment supports or uses the Active Sensing function, although it is part of the official protocol specification.