So attuned is the Slann's mind that he is able to focus power as an extension of his normal bodily processes.
Source: Warhammer Armies Project: Unofficial 9th Edition (v2.2)
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Ability25 points
Once per Magic phase, the Slann may add an additional 'free' Power dice to the casting attempt. This can cause Ultimate Power as normal, and can cause the Mage-Priest to roll more dice than normally allowed.