Here's a cool sequence on the HHI tonight in Hong Kong, highlighting some HFT activity and how I might interpret some of the spikes.

Cool thing about these spikes is prices reacts AFTER the spikes.

Not always profitable or accurate, like the first Buy on the chart below. That's why I don't use this indicator as a standalone buy/sell signal. And it can be dangerous to try to trade alongside these guys during a trend.

But it's really valuable that price usually changes its behavior AFTER these spikes, which makes the spikes predictive to my eyes.