The HEARTBEAT EXTENDER expands the functionality of the HEARTBEAT module. It is connected to the HEARTBEAT module with a small ribbon cable (included) and offers following features:

• six preset buttons to store and recall different tempos with one button press
• MIDI in and out sockets with LED indicators: The HEARTBEAT module can follow incoming MIDI timing data (MIDI start/stop/clock) as an additional option; and the current tempo of the HEARTBEAT is sent out as MIDI timing data if it acts as master clock in a complete system - other MIDI devices will follow its tempo.
• a knob that controls either the swing amount or the transition time when changing from one preset to another (known as accelerando/ritardando).
• a lock switch, that disable the save button and other functions that could be an issue e.g. in live situations.

https://wiki.aemodular.com/pmwiki.php/AeManual/HEARTBEATEXTENDER