Android - How does freezing an app work technically?

Titanium Backup etc just use in-built Package Manager (pm) to freeze an app. pm has a feature to prevent apps from running & from appearing in Launcher. To use it, Titanium Backup etc execute following command:

pm disable {package_name}

You can freeze apps by yourself without using a dedicated freezer app. Just use the above command in Terminal Emulator or ADB Shell.

For example, if you want to freeze Stock Web Browser, open Terminal Emulator & switch to root using su command. Then, execute # pm disable com.android.browser.
It'll return #Package com.android.browser new state: disabled. Done!
Restart your Launcher (some devices may require reboot) to see the app icon gone.

To defrost the app, just replace disable with enable in the command.

I found how pm disable works:

If you run cat /system/bin/pm, it gives:

# Script to start "pm" on the device, which has a very rudimentary
# shell.
#
base=/system
export CLASSPATH=$base/framework/pm.jar
exec app_process $base/bin com.android.commands.pm.Pm "$@"

So apparently it is the same as the PackageManager we use from Java, it is calling it - just in a root context that no user app can directly access.

You CAN check if something is frozen, using

getPackageManager().getApplicationEnabledSetting( the package name ) ==
        getPackageManager().COMPONENT_ENABLED_STATE_DISABLED)