In short, we saw in transition from selling pressure to buying pressure, with a waning of selling preceding the upsurge in buying. The trader seeing this shift in supply/demand was alerted to the likelihood that this was not a trend day to the downside and, indeed, there were many traders leaning short who might need to cover.

Notice also that once we surged above two standard deviations in the $TICK measure (both to the downside in the morning and to the upside during the afternoon), we tended to get follow through of price movement (momentum). Just noticing these dynamics helps keep a trader on the right side of market movement, knowing when to trade a market move and when to fade it.