Why do antipsychotics (which mediate dopamine) benefit bipolars like they do for schizophrenics, what sorcery is this?

Second generation antipsychotic medications are not only dopamine antagonists but also potent serotonin antagonists. Regulating serotonin uptake can help stabilize mood, both with depression and elevated states. So possibly more to do with their effects on serotonin receptors than dopamine receptors. (Not a doctor but nothing I said can’t be confirmed with a quick google scholar search)