Really better if you directly use, as reference, the HP 2920 Switch Series Stacking Technical White Paper available here (not so updated but, in any case, valid) or, supposing your Aruba 2920 Stack is running a recent software version like the WB.16.02 or WB.16.03, the HPE ArubaOS-Switch Advanced Traffic Management Guide for WB.16.03 (which is, in any case, the most update document explaining the stack management for this type of Switch series) at its Chapter 9 "Switch Stack Management" available here.