I watch the SPI and others nearly every day.
There are corners that occur frequently at certain times, but not often enough for me to put trades on regularly.
There is usually a turn before 10am, a frequent turn around 10.05am, another around 1030. Most mornings are south, most afternoons are north, esp. if the morning is south. There are alot of headfakes or shakeouts in the spi, where the first 5-15 minutes continue in the direction of the overnight trend, then it will turn for a countertrend move.
I read that in the US alot of pro traders will turn on their mechanical trading program for afternoon sessions, but trade discretionary methods for the morning. I wonder if this happens in the spi as well, there is certainly more trends in the afternoon than morning. I thought maybe it was funds filling buy orders in the ASX stocks leading the spi for these avo trends. The HSI definitely affects the spi when it opens, as does BHP at times.
When you look at a tick chart, the spi can make a corner for many different reasons.
Volume spikes
DOM pressure
Pivot lines
S/R from previous day/week
News
Sometimes no reason that I can see. Sometimes it will completely ignore these events, and thats the trick.
I hope you find a relationship that's tradeable.