A huge purchase last week in an obscure exchange traded note providing leveraged exposure to U.S. large-caps has mystified trading desks.

UBS AG FI Enhanced Big Cap Growth ETN (NYSEArca: FBG) saw more than 1 million shares traded on May 21. The exchange traded note rally sees more than 50,000 trade on any given day.

Early last week, it appears $600 million worth of FBG shares were created in one fell swoop.

For the week, FBG saw net inflows of $621.6 million, according to IndexUniverse data. It was the top-inflow exchange traded product behind iShares Russell 2000 (NYSEArca: IWM).

FBG delivers 200% of the performance of the Russell 1000 Growth Index.

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