A/N: Trigger warning because basically the whole thing talks about jumping off of buildings (for the sake of his job). Refers to the first two movies but I suppose it's not necessary for you to have seen them. Incredibly short drabble.
Jay can safely say that he's gotten used to jumping off of tall structures. Whether it be the Chrysler building, a fucking huge alien or just some truck parked in the middle of the street, he's got it down. Something about working for MIB for fourteen years (and jesus christ, it's been fourteen years) has given him the ability to just let go and fall. Admittedly the Chrysler building was kind of pushing it, but it was for Kay so he had to. Kay would (and probably has at some point, as Griffin would say) do the same.
The thing that Jay's learned about jumping though, is that there has to be a reason. Without intent he finds himself shying away from the edge, not for self-preservation so much as finding it a waste of time. If there's a better way, he'll find it. So maybe he doesn't get the adrenaline rush, at least everybody makes it out alive.
It's like back in the beginning, when Kay had just recruited him and half of Jay just wanted to go back to being a cop. A lot of stuff went unexplained just being a cop, but it was way less confusing then being a Man in Black, where there was no way he'd ever know everything, even with the seemingly endless resources and the large amount of people who knew more than him. He'd stuck with it though; it's not like he had much to go back to and he could probably make a bigger impact with the MIB.
There had been that first case, with the meteor that crashed and the dude who had turned into a massive cockroach. This was before the dude actually turned into the bug, but apart from being totally nasty, it was the first time he'd had to jump. It was into a dumpster or a pile of trash; he doesn't remember it perfectly anymore, but he remembers how it felt. It was actually an accident, but he ended up pulling a civilian down with him, saving both of them. It couldn't have been more than a twenty-foot drop but every second slowed down and gave him time to analyze the situation; how to twist his body to cushion the fall while still being able to see what was going on around him. It was that moment that made him stay with the Men in Black.
So he's used to jumping, himself. Used to how it feels milliseconds before he crashes, the ground always closing in faster than he'd originally intended. Some days he lives for it, the near-death experiences make him feel alive in a way that even firing a huge gun or catching some alien terrorist doesn't.
Seeing Kay almost fall though, that's different. It's not the Kay he's used to, but it's definitely still him. Still his stupid but funny catchphrases, the same way he looks at O, his miraculous ability to be completely devoid of emotion and everything else he's grown to respect. There's no intent in the way Kay slips, no want, no reason to fall. In fact, if he does fall the world will have ended by the time Jay gets back to forty years in the future. Jay would fall down the elevator shaft a thousand times if it just meant that Kay could get back on top of the platform. He wills it with his mind, ignoring the fact that while aliens are real, magic is not.
And in the end, when Kay gets back up and Jay shouts, "That's my partner! That's my partner!" he means it in more ways than one. Not in the sense like Kay and O should be partners- because they definitely should- but in a manly soul mate kind of way. He's spent almost fourteen years straight driving in a car with the same person, every single day. And yeah, they don't always talk, but he gets that now. He would've fallen down the elevator shaft a thousand times, but more than that, he would've given up jumping. There's no future for him if there isn't one for Kay.