While ECB President Mario Draghi faces his own German-bond-market constraints in his hubristic bond-buying-bonanza, cornering him to taper sooner than later; the Bank of Japan appears to have thrown every textbook out of the window and cranked their plunge-protection to '11', as Bloomberg reports, The Bank of Japan now holds 75% of the nation's ETFs.

Since December 2010 - when The Bank of Japan held no ETFs at all - the central bank has been buying ETFs  (doubling its annual buying target to 6 trillion yen in July 2016) as part of unprecedented economic stimulus. While the Nikkei 225 Stock Average has risen 89% since December 2010, the BOJ’s dominance of the ETF market has raised concerns.

In fact, in a circular vicious cycle, the Bank of Japan’s purchases have helped assets managed by ETFs surge almost 10-fold since the end of 2010 to 25 trillion yen ($230 billion).

As the chart below shows, the central bank now owns three quarters of such funds by market value...

Even the head of Japan’s stock exchange Akira Kiyotasays he is unsettled by the risk of "constant distortion" posed by the central bank’s exchange-traded fund buying.

The impact on Japan’s overall stock market may be limited because the total value of BOJ ETF holdings is the equivalent of just 5 percent of the Topix market capitalization.

But, any tapering of such purchases may erode demand, particularly for stocks in which it is a dominant shareholder.

At this rate, the BOJ could dominate 80 percent of the ETF market by year-end.

Once it decides to stop buying, or even start selling, it’s not clear what the stock market impact will be or who will buy that amount of ETFs.