By matching individual movements, mirror processing provides a representation of body part movement that might serve various functions (for example, imitation), but is devoid of any specific cognitive importance per se. p. 269
Instead, understanding comes from matching higher-order representations of the goals of the action (the quote above continues immediately):
By contrast, through matching the goal of the observed motor act with a motor act that has the same goal, the observer is able to understand what the agent is doing. p. 269
This goal-matching, according to R&S is quite independent of any specific motor act.
...among the neurons in various areas that become active during action observation, only those that can encode the goal of the motor behaviour of another individual with the greatest degree of generality can be considered to be crucial for action understanding... Indeed, parieto-frontal mirror neurons encode the goal of observed motor acts regardless of whether they are performed with the mouth, the hand or even with tools. p. 269
So, mirror neurons, those cells that fire during specific actions such as grasping-with-the-hand and while watching the same specific action -- the very cells that got everyone SO excited -- are not involved in action understanding. Rather, according to R&S, action understanding is achieved by cells that do not code for actions at all, but something higher level, goals/intentions.
It's worth noting that R&S directly contradict themselves in the sidebar definition of "Mirror-based action understanding":
The comprehension of an observed action based on the activation of a motor programme in the observer’s brain. p. 265
A motor program presumably controls a specific action, such as grasping-with-the-hand, not an action-independent goal or intention.
Is the mirror system needed for coding all goals and intentions? No, according to R&S:
This does not mean that the parieto-frontal mirror mechanism mediates all varieties of intention understanding. p. 271
But they want to say that the system IS needed for coding motor goals/intentions. They illustrate with an example:
Mary is interacting with an object (for example, a cup). According to how she is grasping the cup, we can understand why she is doing it (for example, to drink from it or to move it). This kind of understanding can be mediated by the parieto-frontal mirror mechanism by virtue of its motor chain organization. p. 271
It is true that we can make limited inferences about what Mary's intentions are by the way she grasps the cup. But (i) these inferences are underdetermined by the movement and (ii) motor experience with grasping cups is not necessary for making these inferences. Regarding (i), if Mary grasps the handle, rather than pushing the side with her fingertips, we may infer that she intends to drink rather than move. However, Mary could just as well be moving the cup, picking it up to put it in the sink, or picking it up to give to someone. Regarding (ii), simply having perceptual experience with grasping for drinking versus pushing for moving actions will result in the same inferential ability even without motor experience with cups.
Acknowledging the point that action understanding does not require the motor system, R&S site several studies that show that understanding can be achieved without the mirror system. They conclude,
These data indicate that the recognition of the motor behaviour of others can rely on the mere processing of its visual aspects. p. 270
So, to summarize:
1. Cells that mirror specific actions (i.e., congruent mirror neurons), don't support action understanding.
2. The real work of action understanding is done by cells that abstract away from actions and instead code goals and intentions.
3. Intentions can be coded outside the mirror/motor system.
4. The recognition of actions of others can be achieved outside the mirror/motor system.
So what does the mirror system contribute?
...[it] allows an individual to understand the action of others ‘from the inside’ p. 264
What does "from the inside" mean to R&S?
...the observed action is understood from the inside as a motor possibility and not just from the outside as a mere visual experience. p. 270
In other words, it is the "understanding" that I-can-do-that-too and nothing more.
Rizzolatti, G., & Sinigaglia, C. (2010). The functional role of the parieto-frontal mirror circuit: interpretations and misinterpretations Nature Reviews Neuroscience, 11 (4), 264-274 DOI: 10.1038/nrn2805