Picture a sinner with dirt all over him. He goes into a confessional. In the ceiling is a showerhead. Above the ceiling (not visible) is an infinite basin of water; this water is God's power. The sinner confesses, and then with the words "Te absolvo", the priest turns the handle that opens the showerhead. Water (God's power) pours out and all the dirt washes off the sinner.
Originally I came up with this image to explain the different senses in which the "power" of the sacrament comes from the priest and from God. The priest has the power to perform the sacrament, because he turns the faucet. Without him, the sacrament doesn't happen. In another sense, the power is God's alone; he is the one actively doing the forgiving. The priest can't make his own water or wash the sinner off with his hands. Later I realized I could use this analogy for another purpose, too, and wrote the following.
I also want to extend the shower-stall analogy to explain something else that is sometimes hard for Catholics to get across. We really do believe that God is not limited to the sacraments. In some cases, at his discretion, he can choose to cause a rainfall that will cleanse someone of their sins even though they aren't in a showerstall (i.e. forgiveness outside of confession). If someone doesn't have access to a priest, for instance, God can lovingly choose to make up for that lack. Protestants sometimes leap on this and ask then why do we need confession at all, why not just confess directly to God? But for a Catholic, this is like someone walking past a dozen showerstalls (that God has gone to great effort to build so that we might take a shower whenever we have the least bit of dirt on us) and then expecting God to create a special rainfall for them whenever they ask. Such an attitude fundamentally misses the point, although it can be hard to describe exactly where it goes wrong.