Attempting to answer this question made me realize there were a ton of problems with Limb Cavity, and so I have reworked it both for clarity and to scale back its ability to do wild stunlocking via grapple. The new version of the PDF is 12.11.24 and can be downloaded via the normal means.
The intent with Limb Cavity is that it's a burst of unarmed attacks and then you suffer bleed. You can combo it with stuff like Crook's Heavy, and you can use its attacks to grapple as per the normal grappling rules, so it's a little too strong for just a 1 Action 1 bleed cost.
