IMO, it essentially boils down to Lindsay Clancy stipulating/admitting/agreeing or otherwise not challenging the allegations that her physical actions caused the deaths of the three children, thereby eliminating the need for a trial to prove those allegations true. She will admit her body physically acted to kill the children. In exchange for that, her defense wants the only matter "tried" before the Court to be Lindsay Clancy's mental state, i.e., whether she was capable of knowing right from wrong, was in a state of psychosis, or otherwise mentally impaired or insane.
I'm not a lawyer, but the strategy makes sense to me. The prosecution will likely vigorously oppose it if their intent is to try to prove that Lindsay Clancy is a cold-blooded killer who murdered her children with malice aforethought. I don't happen to believe she is, but that's JMO.