ill just post what other ppl have written cuz i gotta go to bed:
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People don't seem to understand how the realization occurred but I explained it in a different thread and people said I did so in a way that made sense.
People's biggest confusion seems to be that Brock was not poisoned from ricin, but lily of the valley, so how was ricin connected to it in the first place?
The ricin cigarette was not stolen to poison Brock, the ricin cigarette was stolen while Brock was simultaneously poisoned by Lily of the Valley, to make Jesse believe he was poisoned by Ricin.
This caused Jesse to suspect Walt, which created a confrontation, which allowed Walt to turn Jesse against Gus, which is ultimately what caused Walt to have the information of Hector Salamanca, which is how he was able to defeat Gus. Walt needed something to occur that would turn Jesse against Gus. The belief that someone poisoned Brock with ricin is what convinced Jesse that Gus was evil.
So, as it plays out, it turns out Brock was likely not poisoned at all, and was instead simply getting into some native plant that showed ricin-like symptoms. This confuses Jesse, as the timing of the poisoning combined with him simultaneously losing his ricin cigarette really raised some suspicions. This caused him to go nuts and tear his house apart looking for the ricin cigarette, because the whole situation seems very fishy.
Walt places a fake ricin-containing cigarette in the roomba, clearing up that loose end of Jesse's doubt of the situation. It's obvious Jesse still has his doubts that such a major coincidence could occur that ultimately caused him to go against Gus.
So, flash forward to Huell lifting the weed, Jesse suddenly realizes that the ricin cigarette WAS lifted off of him (also, keep in mind that we watched season four nearly 2 years ago, but the event only takes place about 10 episodes ago at the end of season 4.) The significance of the ricin being lifted off of him is that it WAS NOT a coincidence that Brock was poisoned by lily of the valley when he "lost" the cigarette, meaning that him losing the cigarette was a necessary part of the plan for Walt to manipulate him.
Jesse realizes that Walt was playing him. He may not know the full extent to why or how, but he realized that the cigarette he "lost" was actually taken from him, and that's enough for him to realize he was being played."
In case anyone is wondering, here is the dialogue from the season 4 episode "End Times" where Jesse figures out exactly what happened, but Walt talks him out of it. This is how Jesse came to the realization so quickly in this episode, because he already figured it out before:
JESSE: I had it. I had the cigarette with the ricin in my pack this morning. The last time I saw Brock was last night. And this morning I switched the cigarette into a new pack. There's no way Brock took it himself!
WALT: Jesse, you're not thinking clearly, listen, you said it yourself, that you had it this morning. Then when could I have possibly gotten it?
JESSE: You...you had Saul do it. Yeah. Yeah. I went to his office. He called me in, just had to see me today. His big man mountain bodyguard patted me down, that's when he must have stole it off me right? That's the plan, was that the plan?
see now THATS a good explanation. u and hexagram cleared it up for me. basically walt wanted to create that confrontation and prayed that he could change jesse's feeble mind which he did. it was a huge risk but it worked out.