jasima is just as much a 'semantics modifier' as words like ala and taso. it has meaning on its own, but it has special modification rules for the word behind it besides adding its meaning to it: it "opposites" that word (like how ala negates, or taso onlyifies)
so the semantic space reverses in some way. vs ala where the semantic space becomes "all except this" or "does not do this"
this is rare. typically antonym words or "X ala" are used instead, but sometimes these are not available.
take sin. if you are under oath to only use nimi ku suli (so no majuna), then using "sin jasima" might be interesting to use.
but this can imply extremes (akin to "he is the opposite of young" meaning "he is an ancient fossil" rather than "he is old")
this is more common and more understood. in "X li Y jasima e Z", instead of X doing Y to Z, Z is doing Y to X. or in "X li Y jasima", Y happens to X instead of X does Y.
this is akin to the passive voice, focusing on the event happening to X while not implying someone (like Z) did something to have Y happen. there's a sort of invisible ijo that is doing Y to X. or the Y is happening in the opposite direction that it normally does.
"jan li pakala jasima". the person was broken.
This is also common, commonly overriding opposite #2 for ditransitive verbs. What ditransitive verbs means is that for certain words used as verbs, the full structure can be ditransitive taking an indirect object using tawa: the Subject (li) does something with (e) the Direct Object (tawa) to the Indirect Object. Both objects can be left out, creating holes in the sentence which jasima can help patch up without adding ijo everywhere.
Here, jasima turns the phrase "X li Y e Z tawa W" into "W li Y jasima e Z tawa X". X does Y with Z to W becomes more like W was Y'd to with Z by X.
An example is toki. "jan li toki e moku" is not "the person talks to the food" but "the person talks about food". The person who is being talked to is referred to with tawa. "jan li toki tawa sina." is "the person talks to you".
Therefore, "jan li toki jasima" would not mean "person was talked about" but "person was talked to".
Again, this might be good if you want to focus on the event, while avoiding the idea that the person has done something to have Y happen. A good example is with pana jasima. "jan li pana jasima" means the person receives. but it does not imply the person did something to receive it (that would be taking: lanpan) and does not focus on the end state (kama jo)