It doesn’t dampen the effect of El Niño, it’s the opposite phase of the El Niño/Southern oscillation climate system. Both phases can have bad and good impacts on different parts of the world. This year Utah (where I live) is having a close to record snow, that is probably due in part to La Niña. This is good for us. Interestingly El Niño can do the same thing for us here as well.