An orgy of last-minute AAPL trades

Someone sold 6 million shares at $348.24 as Apple put the NASDAQ-100 rebalance behind it

Shares fell sharply before Friday's closing bell

Apple investors had been dreading Friday’s close since early April, when NASDAQ announced that starting May 2, Apple’s share of the NASDAQ-100, one of the world’s most heavily traded stock indexes, would be reduced from 20.5% to 12.3%.

All 100 stocks in the index were to be affected by the so-called Special Rebalance, but none as much as Apple (AAPL), whose market cap had grown so large it was starting to make the NASDAQ-100 wobble like badly weighted wheel.

Fund managers could try to anticipate the effect of the 8.16% drop, but the actual realignment of their portfolios had to wait until Friday’s close.

The result was a record-breaking orgy of trading, and not just in Apple. According to NASDAQ, a total of 329.11 million shares worth $12.7 billion changed hands in the blink of an eye — 779 milliseconds to be precise — at the Closing Cross, the moment when the major exchanges reconcile outstanding orders and set their closing prices.

But the action in Apple began before the closing bell and continued for several hours afterward.

In fact, someone was selling off Apple shares at high volume on April 1, two trading days before the Special Rebalance was announced, suggesting to many that news had somehow leaked.

In Friday’s market action, Apple’s shares rose like a souffle — up $7.2 (2.1%) at one point — only to collapse nearly $4 in the last 12 minutes. By the close, 35.9 million shares had changed hands, double Apple’s average volume, most of them in those last 12 minutes.

Then, after the closing bell, more trading in Apple — 8.36 million shares worth.

NASDAQ’s record of those after-hours trades goes on for 95 pages, most of them filled with small high-frequency trades of 100 and 200 shares each.

But there were some monsters as well. The biggest we found occurred at 5:29 p.m., when someone executed four Apple trades of more than a million shares each at $348.2376. Total transaction: 6.094 million shares worth more than $2.1 billion.

Let’s hope whoever it was has got that out of their system.

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[Follow Philip Elmer-DeWitt on Twitter @philiped]