The reason on why downloading this app is you can install cracked apps for your iPhone, since the ipa.files can be downloaded and installed manually without the need to launch iTunes and pay Apple for the files.
This newest release brings bug-fix to solve the problems found on the previous version, AppSync 4.0. To get the AppSync 4.1 you must go to Cydia, but before move forward, your iPhone need to be jailbroken. Follow this link if you use iPhone 3GS:
Jailbreak iPhone 3GS On iOS 4.1 using Pwnage Tool (old bootrom)
Or if your iDevices is iPhone 4, you are forced to wait until SHAtter iOS 4.1 jailbreaking tool is released
Then follow these steps to install AppSync 4.1 for jailbroken iOS 4.1 iPhone And iPod Touch1. Launch "Cydia",
2. Tap on "Manage"
3. And then tap on "Source":
4. Hit "Edit" to "Add" the url source. Then drop this url http://cydia.hackulo.us, and then "Add Source"
Cydia will do the automated steps in order to automatically update the source.
5. When it's finished. You can search for AppSync for OS 4.1, then install the app. Now you have the ability to install .ipa files manually on your iOS 4.1 iPhone