Most iPhone users watched in awe when Motorola's Moto X gained always on voice Control. You could say OK Google and then simply issue your command. Without ever touching your device. Then Google created a new launcher that allowed you to voice control, your device anytime you where in the launcher.
It seems iPhone users were left out in the cold until now.
Before you get too excited, it is important to remember that OK Siri is a hack for jailbroken iPhone's only. If you have any security concerns, you should not jailbreak your device.
Once you install the hack, you have to turn on the always listening mode in the Activator settings (look for OKSiri). A little microphone icon then shows up in the status bar and the device is listening for your commands. You start any command by saying OK Siri.
PRactically you'll have to wait a fraction of a second between saying OK SIri and issuing your command (to give the system time to react). Also once you have performed the desired action and press the home button to come back into IOS land, the OK Siri agent will need a couple of seconds to re-initialize in the background.
Motorola's MotoX was great because the phone is always listening, even when the screen is locked. OK Siri does the same which means you can issue commands even to a locked phone (which is a nice touch).
So it is an interesting addition but not without its problems. For an unexplained reason, the agent seems to freezes sometimes which disables the always listening functionality. If you wait a couple of minutes, it comes back all by itself.
The speech recognition of the agent is no where near that of Google or Apple. This means you may need to repeat your command a couple of times.
You can download this hack from the Big Boss Cydia repository right now for free. Ultimately I am a security nut and will NOT jailbreak my main phone because it opens up too many security vulnerabilities but if your device is already jailbroken, you can take this add-on for a spin. If you don't like it, you can just remove it.