Shortly after the release of the second edition of Dungeons & Dragons, its company T.S.R. released a supplement introducing psionics.  Like yoga instructors writ large, a psi possessed "introverted" abilities such as levitation, teleportation, shape-shifting, and mind reading, and served as a contrast to the sound and fury of clerics and wizards.  However, the supplement was not well received among players, who deemed the role unbalanced and at odds with Tolkien's milieu.  Later supplements unsuccessfully attempted to correct the deficiencies.  After that, some acquaintances and myself began our own public attempt with The Skills & Powers Psionics Netbook.

We re-examined the abilities that effortlessly allowed invading castles, escaping imprisonment, and dominating antagonists, yet wizards and thieves could do these things regularly.  We considered the problem may lie with the abilities remaining unseen as they were in use, but adding fireworks or requiring tools turned the psi into just another wizard.  In the end, we offered up all manner of rule changes, ultimately letting the individual gamemaster decide.  We had failed to discover the real problem, and chose a buffet approach by default.  

Then, over the course of the following months, I began to receive long and joyous emails from gamemasters who had completely turned their feelings around on including psionics in their campaigns.  But none of this was due to the mass of proposed rule changes.  Rather, during its creation, I had adorned the netbook with a variety of new items, new abilities, and new monsters. One purely decorative power that had spontaneously popped into my head during writing was Animate Tattoo.  I had had good experience giving a player of mine a magical item for pure decoration, a magical cloak that could at will ripple as if in a wind.  So I added Animate Tattoo and promptly forgot about it.

It was this inconsequential little power that caught the gamemasters' eyes.  They would write to me of entire cultures and rulesets they had created pertaining to tattoos and psionic abilities:  how tattoos affected psionics, how psionics depended on tattoos and tattooing, how various attitudes, reactions, and occupations grew around tattoos as a result.  I heard just about every imaginable way of negating psionic abilities by scratching, damaging, burning, or touching all manner of energies, fluids, and materials to a tattoo.  Finally, antagonists could identify and neutralize a psi on the street, could throw him into a cell, and could keep him there.  

That was how psis disrupted the game despite average battlefield performance.  A psi that can never be identified, captured, and immobilized is largely immune to common plot devices.  Just as rules define and circulate the various resources of gameplay, plot requires certain resources to function.  The unassuming and variable appearance of the psi excoriated those resources, so the plot starved.  We were looking for a numeric rule to trim, but we needed to add a fictional rule.

If the distinction isn't clear, imagine if the cast of Harry Potter did not need wands to cast spells.  They only point a finger and bark a word.  Now remember the numerous times that disarming, losing, or capturing a wand occured in the books.  It allowed one to capture a wizard without physically beating him or her senseless, to blackmail a wizard so their wand would be returned, to reduce or enhance one's abilities, and even to characterize the owner, as with Ron Weasley's taped-together hand-me-down.  Removing the rule of "wizards require wands" from the fiction may not affect the power balance between the various characters, but it certainly impedes plotting.  And a gamemaster frequently must create plot events on the spot. 

While a gamemaster typically customizes his world with some new cultures or house rules or such, it is practically universal that wizards wear robes and need spellbooks, that clerics wear holy symbols and vestments.  By contrast, the psi's only pre-packaged characteristic was they didn't need to point or speak to use their abilities, and to change that makes them wizards again. The gamemasters who had written me seized upon the idea of tattoos as the defining and necessary characteristic of psionics.  It allowed plot hooks to function again -- it was a plot device itself -- but it also preserved the distinctness between the psi and wizard without disturbing the already-balanced numeric rules.  And as a bonus, by way of tribalism it grounded the psi in the pseudo-medieval milieu.  Those gamemasters found nirvana in adjusting fiction's ruleset.  They attained interactive narrative.