Hello Leo,
As the hardware guide states, the XGM, VIM, and SummitStack modules on the SummitX product line are NOT hot-swappable. You will need to power off the switch before installation or removal to ensure proper operation and to prevent damage to components.
You mentioned the X450, which does not have a removable stacking module. You'll find stacking modules for the X460, X480, X650, and the X670V. The X440 "non-10G" models have built-in stacking ports, as do the earlier SummitX products (X250, X450, X450a/e).
-Drew