Compressed air, but you're not blowing it into the sensor.
It's to charge the brush so that it picks up any particles on the sensor. You can also use a rocket blower to charge it.
Read more about it here:
http://www.visibledust.com/index.php
No issues with sensors. I've used it a number of times on 5 different DSLR bodies over the years and haven't had an issue with any of them.