AppSync is updated to AppSync 4.2 for iOS 4.2.1 jailbroken iDevices. For those of you who do not know about AppSync, lets explain a little, this tweak patches Mobileinstallation file on iOS devices which let you install .IPA files manually by avoiding iTunes and Apple charges.
AppSync 4.2 also makes possible to install crack applications on jailbroken iOS 4.2 iDevices.
How to Install AppSync 4.2 on iPhone, and iPod Touch with Jailbroken iOS 4.2.1
1- Launch Cydia on your iDevice
2- Press Manage tab > Sources
3- Go to Edit > Add
4- It will ask to provide a URL so type http://cydia.hackulo.us and hit Add Source
5- Now wait for a while, when installation process will get finish, go to Cydia and search AppSync for OS 4.2 and install that, it will patch Mobileinstallation file to your iDevice running over iOS 4.2.1.