Nothing can rattle someone's cage like an ex-spouse showing up unexpectedly. That's a fact that Jimmy McGill is fully aware of, which is why he invites Chuck's ex-wife Rebecca to come to his bar hearing during the May 8 episode of Better Call Saul. But the circumstances are much more loaded than that since a flashback at the beginning of "Chicanery" shows that Chuck never told Rebecca about his illness. By having Rebecca attend the hearing on Better Call Saul, Jimmy exposes Chuck's greatest vulnerability while he's on the stand — and it leads to Chuck reacting exactly the way his brother wants him to.
Viewers of Better Call Saul were introduced to Rebecca — guest star Ann Cusack (of the famous Cusack family) — in Season 2's episode "Rebecca." In a flashback, Chuck was experiencing marital bliss with no electromagnetic hypersensitivity. His younger brother had just moved to New Mexico and though Chuck was worried his violinist wife wouldn't like Jimmy, she was more amused by him than Chuck ever was — something that, of course, threatened Chuck. In the "Chicanery" flashback, Rebecca and Chuck have separated and she doesn't know of his sensitivity to electricity, so Jimmy helps Chuck come up with a ruse to hide his condition from Rebecca. Besides this setting up what happens in the episode perfectly, it also highlights how accommodating Jimmy was to his brother over the years despite their tense relationship.
When Rebecca attends Jimmy's hearing with the New Mexico bar association at the behest of Jimmy, Chuck isn't too happy about it. As she says, she came to the hearing because she thought it was the right thing to do, but she realizes she's more of a distraction by Chuck's hostile attitude toward her. Yet, that's exactly why Jimmy invites her in the first place — to put Chuck on edge before his cross-examination even begins. Chuck's fully aware that is Jimmy's intention, but that doesn't mean he is able to control his emotions.
Chuck had been prepping for his big appearance at Jimmy's hearing by practicing how he would say he loved his brother — you know, something that shouldn't require any practice. Yet, Jimmy is able to still pull one over on the overly-prepared Chuck by having Rebecca show up. Although Jimmy doesn't explicitly state it in "Chicanery," there most definitely seems to be a correlation between Chuck's separation from Rebecca and his illness — or, at least, that's what I assume Jimmy thinks since he is one of the only people to openly state that Chuck's condition is mental, not physical. Even if his alleged electromagnetic hypersensitivity wasn't brought on by his divorce, Chuck is still incredibly protective of this part of his life. So Jimmy manages to reveal Chuck's biggest weakness in a way that he can no longer hide it from Rebecca.
All of this works out as Jimmy planned it to since Rebecca attending the hearing, combined with Huell planting a cell phone battery on Chuck, leads to Chuck breaking down on the stand and revealing his true intentions behind recording Jimmy's confession. (And Huell's actions also kind of prove that Chuck's illness is mental.) While Chuck — and Rebecca for that matter — may never forgive Jimmy for what he does to Chuck, Jimmy's trick most likely will allow him to continue practicing law. Because while Chuck is just as conniving as his brother, Jimmy using his brother's ex-wife shows that the youngest McGill will always succeed more when it comes to chicanery.