API UseAction

Perform the action in the specified action slot. UseAction(slot [, checkCursor, onSelf])

Arguments

 * slot : Number - The action action slot to use.
 * checkCursor : Flag (optional) - Can be 0, 1, or nil. Appears to indicate whether the action button was clicked (1) or used via hotkey (0); probably involved in placing skills/items in the action bar after they've been picked up. I can confirm this.  If you pass 0 for checkCursor, it will use the action regardless of whether another item/skill is on the cursor.  If you pass 1 for checkCursor, it will replace the spell/action on the slot with the new one.
 * onSelf : Flag (optional) - Can be 0, 1, or nil. If present and 1, then the action is performed on the player, not the target. If "true" is passed instead of 1, Blizzard produces a Lua error.