While its 'Shop the Look' Pins identify specific items within any Pin image and connect users to purchase pages for each relevant product, the new option Pinterest is working on will look like 'Complete the Look', thereby accounting for the products users have searched for so far, providing related recommendations based on relative trends and other factors.
According to Pinterest:

The new product makes wide sense of scene context for suggesting visually compatible results in Fashion and Home Decor Pins. Complete the Look takes context--like an outfit, body type, season, indoors vs. outdoors, numerous pieces of furniture, and the general aesthetic of a room--to power taste-based recommendations across visual search technology.

As can be observed from the examples given above, the algorithm essentially includes its suggestions to visually similar or contextually related items within its recommendations, considering the parameters described above.

Pinterest trained its model off a dataset of images that included fully constructed scenes of varying degrees of product matches. It cropped out the actual products it was interested in for this particular image so the system was able to recommend similar matches based on what it learned were relevant partners. As with all algorithm systems, it is fairly technical, but you can read more about the process specifics here. 

Based on this initial training model, Pinterest's systems can now identify relevant product matches based on image context - here are some examples of its contextual recommendations.
The process will help connect even more Pinterest users to relevant product matches and recommendations, which could help to further fuel its growing eCommerce ambitions. And with Pinterest leading the social media pack in terms of product discovery, the addition could prove significantly beneficial for users as they seek out just the right companion pieces in various contexts.

Complete the Look is being tested in-house and then will be released to the recommendation tool set of Pinterest.