Key points
- SanDisk (SNDK) was down on the day at 3:15 p.m. ET, then closed up 5.7% at $1,569.61.
- SanDisk was the largest addition to the MSCI World Index, effective at Monday's close, so index funds had to buy at the closing price.
- The final 10 minutes carried 1.93 million shares, roughly 17 times the volume of the preceding five-minute bar.
SanDisk (SNDK) traded at $1,460.52 at 3:15 p.m. ET Monday, below Friday's $1,484.98 close and down on the day. Over the next 45 minutes volume surged, the stock ran, and it finished at $1,569.61, up 5.7%.
The reason is an index change. SanDisk was the largest addition by market capitalization to the MSCI World Index in MSCI's August review, ahead of Carpenter Technology and ATI, and the change took effect as of Monday's close. Funds seeking to track the index closely needed to establish their positions at or near the closing price.
How the buying arrived
| Time (ET) | Close | Volume |
|---|---|---|
| 3:15 | $1,460.52 | 76,562 |
| 3:30 | $1,473.62 | 135,610 |
| 3:45 | $1,489.62 | 111,258 |
| 3:50 | $1,545.79 | 696,779 |
| 3:55 | $1,569.61 | 1,228,492 |
The volume column is the one that matters. From 3:15 to 3:45, the stock drifted up about 2% on ordinary afternoon turnover. During the final 10 minutes, it added another 5.37% as 1.93 million shares changed hands. That was roughly 17 times the volume of the preceding five-minute bar and about one-twelfth of the entire session's volume compressed into 2.6% of the trading day.
SanDisk turned over 23.2 million shares Monday against a float of 144.3 million. Monday's volume equaled about 16% of SanDisk's tradable float.
That concentration is the signature of index buying. A fund tracking MSCI World is measured against the index's closing value, so it needs the stock at the closing price, not at 2 p.m. and not at some better price it might prefer. It files a market-on-close order and takes whatever the auction gives it. Multiply that by every passive fund benchmarked to the index and the demand lands in one window, on a deadline, with no price sensitivity behind it.
