On the Hollow’s Mask
The mask has a dual function: it hides and manifests. These functions are opposite, yet inseparable.
The essence of the hollow is its loss of heart (loss of love, purpose and authenticity) the domination of drives, animal insticts as motivation, and insatiable desire to fill the void in their hearts. Yet an ordinary hollow, being an in-authentic being, cannot admit that this is his state. No. He puts on a mask. This covers his shameful insticts. But most of all, he can say: “Look, it’s not ME who does all these nasty things. It is this guy in a mask – this strange persona”. Person comes from Latin persona, which comes from Greek prosopon – theatre mask. The person who is symbolized, manifested by the mask is a different from the person who wears a mask.
So the mask symbolizes The Hollow, so to say. That is the role (note that ordinary hollows sooner or later lose their individuality, since they all get immersed in playing the same role). A hollow, by putting on the mask of The Hollow, tries to hide – from himself and others – that he is what he is, namely a hollow.
So the mask is a source of hollow’s power in 2 ways. First, it enables them to function as hollows – once it is broken (cf. Orihime’s brother), the hollow has to face his human nature and his miserable state. Then he can’t behave as an ordinary hollow. Secondly, the mask symbolizes and realizes the form/persona of The Hollow. By wearing the mask, one assumes this form.
This is what happens with Vizards. Once the mask is on, they assume/manifest this from of their being and hide their individuality (and, remember, Hirako says to Grimmjow: “it doesn’t matter who I am”). However, in this form they can put the strength of their WHOLE being, including their heart.
And that is why they are certainly stronger than the Arrancar. Arrancar are hollows which reached the stage of self-awareness and authenticity. They have their human individuality. But they do not hide their hollow identity: the fact that their motivation and life is that of raw instinct. They are shameless hollows (though, not quite, or not all of them. The more sensitive seem to realize that their current state is lamentable, born from fear. So, on the one hand, they are very sensitive to humiliation; want to prove that still, they are better. On the other hand, they follow Aizen on the path to glory and perhaps liberation). The broken mask means that the border between the outer form/the mask and the inner essence has been broken. The same thing – the hollow – is both outside and inside; the actor and the persona coincide. They cannot totally get rid of the mask, for that would mean that they were unrelated to the hollow form – which is false. So when the mask is forcibly torn off, they lose contact with hollow powers. Then, in fact, their appearance is deceptive – they are like monkeys who are made to look like humans. But since this is false, and Arrancar are authentic beings (or at least more so than ordinary hollows), the mask soon grows back (in fact, the mask is always “somewhere”. When it is put on, hollow form/powers are manifested. When it is not worn, the hollow form/powers are hidden).
But still, Arrancar are imperfect and contradictory beings. The broken mask is a symbol of that. That’s my intuition, I’m not sure how to develop an arg. for that.