Since the Institutional Investors are the largest stock owner group (owing over half of the equity value), when they sell, they put tremendous downside pressure on the market.

When the don't sell, the lack of their selling takes pressure off of the market and allows it to go higher on retail buying (even if Institutions are buying very little).

So, what are they doing now? If you look at the chart below, Institutional Investors were still in a selling down trend (a positive) at the close yesterday which has been a market positive ... but note that the red Selling trend line will be testing the blue trend (resistance) line in the next few days.