Since the Institutional Investors are the largest stock owner group (owing over half), 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 today's posted chart, Institutional Investors were still in a selling down trend at the close yesterday which has been a market positive.