When slotted on either bar, you focus with lethal intent, causing your Light and Heavy Attacks to generate a stack of Relentless Focus, increasing your Weapon and Spell Damage by 80 up to 5 times. Fully-charged Heavy Attacks grant two stacks. When at full stacks, you can consume them all to fire a spectral arrow to deal 19754 Disease Damage, and heal for 33% of the damage dealt if you are within melee range.
OK, well I found an answer - apparently it has been like this for a while (I only recently started using this again after Concealed Weapon got nerfed in Update 41).
/en/discussion/651251/pts-patch-notes-v9-3-0
Why does ZOS keep trying to stop us from ganking from stealth - is that not a valid gameplay?