Passion iPhone app measures your sex performance by noise+movement. So instead of putting your iPhone next to the bed you have to put it somewhere where there's movement (and noise). Usually you're pretty buff nekkid having sex, so there's no other option than to shove it up your, or your partner's ass to get accurate measurement. Just be sure to have the microphone end stick out, otherwise the noise will be muffled.
Source: gizmodo