How do helicopters avoid detection from enemies by using IR suppressors?
An IR sensor detects objects that are at a higher (or lower but that's not how missiles work) temperature than the surrounding environment, most of the helicopter won't be that much hotter or cooler than the air around it, so wouldn't distinguish it much unless you get very close. Jet engine exhaust is much hotter than the surrounding environment, so it shows up very clearly to an IR sensor from far away. 

A suppressor diffuses hot exhaust into a larger air mass, lowering the temperature of the exhaust compared to the surrounding air before the sensor can see it. Lower temperature means less heat for a sensor to see.  A suppressor also makes the infra-red picture fuzzier, i.e. less of a distinct point. The lower temperature and fuzzier image decreases the effective range for a sensor, meaning it would have to be closer to detect it. Suppressors often deflect exhaust, pointing it away from where a sensor would be expected to read, or shield the exhaust. 

Suppressors don't prevent an IR detection in the same way that stealth doesn't prevent radar returns, but a suppressor means you need to get closer.