This illustration is designed to show that faith is not a work. It helps me to think about the relation of grace and faith, as in Eph 2:8,9.
Think of a milkshake. God graciously gives the milkshake. We didn't provide the resources, or make it. God did. But we are supposed to drink it.
Some people would say that our faith-response to God's gracious gift is the straw through which we suck up the milkshake. But I don't think so. God gives the straw as well.
So then faith must be our ability to suck the shake up the straw to ourselves. But I don't think that is right either. We can't even suck without God giving the ability, and even then, I don't think that is where faith comes in.
I think faith is more like God doing everything, providing the ingredients, making the shake, giving the straw, drawing the shake to our mouths, and the response of faith is merely savoring what God has done.
I think that is what faith is. It is realizing that without God, we can not be saved, and that we cannot contribute to the process. Faith is savoring, appreciating, and valuing what God has done in Christ. Faith is savoring Christ. Is it essential to salvation? Yes. But it does not place our actions at the center. God through Christ is at the center.