This spell cannot be used during combat. Hearthstone can be used during combat, but it will be interrupted if the user takes any damage.
Typically the current expansion's main hub will contain portals to major cities. By setting their hearthstone to the main hub, shamans can hearth to the hub, portal to their desired main city to do business, and then astral recall back.
Patch 1.3.0 (07-Mar-2005): The tooltip now includes the home point the spell will recall you to.