Similar to what Yahoo! search advertisers were able to use within the analytics tools (Full Analytics and Conversion Only) on the Yahoo! Search Marketing platform, the Assist metric is now available within Yahoo! Web Analytics.
The assist* metric reveals which campaigns (of any type) most often indirectly contribute to the conversions you value in order to better evaluate marketing opportunities.
The metric is calculated based on the number of times a campaign drives a visitor to your website prior to the source credited with a conversion.
Assists are a campaign-based metric available in the Custom Report Wizard and are treated as any other campaign-based metric. They can be aggregated at higher level categories or viewed for each individual campaign being tracked.
All assists occurring within 45 days of the conversion event will be recorded. Note that a single marketing activity can assist multiple conversions.
Please also note that individual keywords or other organic traffic sources are not tracked as assists.
As you can probably tell by reading the above description, the Assist metric is very valuable for helping you understand whether or not your camapigns (display, search, email, etc) contributed to the conversion of another campaign.
For example, you'll now be able to see that while your display ad did not convert very well on its own (directly) it may have contributed (assisted) in driving conversions for other campaigns (such as search). Over time, of course, you'd be able to make smarter budgeting decisions for your display campaigns. The same goes for other campaigns as well.
*Note: Currently the assist metric only provides data on clicked campaigns. It does not provide impression assists.