The Torah tells us to open our hands to our poorer brethren. Not just to give to them – not just to make sure they are taken care of – specifically, to open our hands.
Why are hands a metaphor for giving ( aside from the obvious reference to the limb that grabs hold of what we have ) ? Because when the fingers are closed they all seem similar in size – only when they are open do we notice that no two are alike. Similarly, when we consider those who are in need – we may stumble into the fallacy of assuming that they can all be taken care of in the same fashion and that the same basic goods will suffice for them. However, as chazal teach us – one must give to the poor "די מחסורו"- i.e. that which he is lacking. And since we are all different – so are our needs. [quoted in the sefer פנינים משולחן הגר"א ]
There is also another angle from which to understand the role that the hands play in giving tzedaka.
Most other limbs are naturally open, or straight. When the muscles are relaxed – your legs lie flat and your arms are completely extended. Your fingers however, are another story. You must consciously flex your muscles to fully open your hand. This can be seen as an allusion to the fact that we are by nature selfish – we must overcome this inborn trait and learn to "open our hands".