When Yahoo! detects a hot topic being queried on their search engine, their algorithm will select Tweets to present in the results.
When Tweets do appear, they'll usually be placed at the bottom of the results.
Image via Yahoo! search blog
Optimising Digital Marketing Campaigns with Search, Social and Analytics
At SES London (9-11 Feb) you'll get an overview of the latest tools, tips, and tactics in Paid, Owned, Earned, Integrated Media and Business Intelligence to streamline your marketing campaigns in 2015. Register by 31 October to take advantage of Early Bird Rates.