There are two major changes.
Yahoo was responsible for the sales relationships for Bing search ads under the partnerships previously. All of those sales relationships are going to come under Microsoft in the coming months.
The new deal now only requires Yahoo to pull a majority of its traffic from the Bing ads marketplace. Just 51%, rather than the 100% it was required to pull before. That means the other 49% of Yahoo’s search traffic could be monetized by Yahoo’s own ad units or by another search provider, like Google.