Banking holy power like this is only recommended on this fight when you're not actively tanking. It can be done while actively tanking, but you will (by design) have to choose not to use a SoTR for the previous shadowy attack. It can also happen by accident just through lucky procs, but that is not 5 Holy Power between a shadowy attack as much as it is just generating Holy Power slightly faster than you need it (attack-4HoPo-attack-3 HoPo-attack-4HoPo kind of thing).