Echoes of Power
Echoes of Power is a temporary SP regeneration that begins once a character drops below 12 SP. It will restore 4 SP every 6 seconds until you have at least 12 SP again. The effect can put you over 12 SP if you have more than 0 when it begins to regenerate your SP (for example, if you start with 3, it will recharge to 15).
Finally, the Endless Faith epic destiny power grants the Echoes of Power feature with SP targets of 18/24/30, rather than 12, depending on the number of ranks taken.