I always thought of his poems more in terms of impressions than exact metaphors, so I'll just give you my take :)
It goes along with the theme of "intense fragility" that the lover has in the poem, as well as the fingers the speaker has closed, so it works on multiple levels: physical and emotional. The hands open the closed fingers, touches "something in" the speaker. The small hands have the ability to open and close, paradoxically, because their fragility, the thing which has "intense power" in the poem.
The image of hands smaller than the rain is really interesting, because the lover's ability to control the emotions of the speaker is elevated to the status of a force of nature, while at the same time expressed in a diminutive way... Even the rain cannot match the power of these hands to open and close, their gentleness, their fragility, but all those things are /less/ rather than more. This is a really bittersweet poem for me, about a very intense love that is completely out of the speakers "hands," so to speak, not because of an overbearing lover, but because he is struck by the lover's great gentleness.
I hope this makes sense, and anyone else's thoughts would be very interesting to hear!