Durex U.K. has launched an interactive experience to promote the launch of its first e-commerce offer -- and as you might expect, it's steamy. But it's also clever.

The TV ad, shown here, depicts a mail carrier delivering condoms to an apartment block where everyone appears to be getting it on -- glimpsed through windows, or even on the doorstep. However, if you download the app, available via iTunes or Google Play, and point your phone at the TV or computer screen where the ad is playing, it synchronizes to reveal the film's alternative view on the second screen -- so you get to see what's going on behind those closed doors. (It works by using the mobile device's camera and microphone to detect that the ad is playing, synchronizing the second screen to the exact corresponding point in the video). Havas Worldwide's London office developed both the app (which functions as a mobile version of the e-commerce site, durex.co.uk) and the commercial.

Durex is one of a series of marketers to use technology to show an alternative version of an otherwise straightforward ad; Honda recently made waves with "The Other Side," where viewers can see a different version of the same events by holding down the "R" key on their keyboard.