According to their paper the Georgia Tech team wanted to create code that could be rearranged after it had passed AppStore's tests. The code would look innocuous running in the test environment, be approved and signed, and would later be turned into a malicious app.
They created an app that operated as a Georgia Tech “news” feed but had malicious code was distributed throughout the app as “code gadgets” that were idle until the app received the instruction to rearrange them. After the app passes the App Review and lands on the end user device, the attacker can remotely exploit the planted vulnerabilities and assemble the malicious logic at runtime by chaining the code gadgets together.
The instructions for reassembly of the app arrive through a phone-home after the app is installed.
The app will run inside the iOS sandbox, but can successfully perform many malicious tasks, such as stealthily posting tweets, taking photos, stealing device identity information, sending email and SMS, attacking other apps, and even exploiting kernel vulnerabilities.