Standard Client update chunks tell the server your current position. You send one every UDP packet. Action chunks tell the server what action you're going to perform (for instance, casting magic).
Currently SE uses your position from the previous UDP packet as the start time for your current action. So:
UDP packet 1: Standard Client chunk position 1
UDP packet 2: Standard Client chunk position 2 + Cure IVd action packet
Cure IV would be treated as if you started casting it at Standard Client chunk position 1 for interruption purposes. This is why you have to stop and wait half a second before beginning to cast anything on mage jobs. If they switched it to use the standard client chunk position in the same packet as the action, you wouldn't have to do that one-count pause anymore and mage jobs would feel much more active because moving would be less of a penalty.
That's what I wish they'd fix.