How to build an app (packaged as IPA file) that can install deb package on jailbroken iPhone

Question

I downloaded an IPA file from http://m.qq.com/ , and install it to my jailbroken iPhone with iFunBox, run and after some self configurating, it rebooted automatically, and when the device restarted, a deb that was built in that IPA file was installed.

I knew that installing deb packages requires root privilege, but the internal executable in that IPA file is normal application, so it made me confused.

PS: SBSettings is not installed on my iPhone, so the answer cannot solve my problem.