Ostensibly I am talking about a set-up here where short(er) term participants are testing to see where value lies from the long(er) term participants and therefore trading a reversal which is often (but not always) in line with a trend on a higher TF.
Think about how a trend often starts. There is a sell-off and for a few days after there are attempts to push lower. However you see that although there is higher selling activity (look at volume), the price is trading in a range. This is long term participants building a position. Then once all the sellers have gone, what is left? A demand/supply imbalance. Then price starts to go up and then momentum traders get on the bandwagon and add fuel to the move. Who is selling into them all the way up? Is the the participants who built the initial position? To do this you need a lot of money operating over longer timeframes.
In control means exactly what is says - are the long term traders in control in the current session or the short term traders? Yes it is quantifiable as you can see it in the form of the value areas that are created.
With the indices, futures market often leads the cash market, especially on S&P500 IMO.