At first... At first, you must know it works for all weapons, but it isn't looks that great, because inspect animations are ONLY good looking on karambit.
Also, people who spectate you CAN'T see when you are spining it, because this trick is some sort of bug.
[Small Update] Actually, butterfly inspect animation also can be used to ♥♥♥♥ your mind, so enjoy it :)
How to: OK, it's really simple, you just tap F, and then R, and repeat.
At the end... My friend showed me this trick.
I can tell it's pretty popular on servers where you can choose your knife.
That's all to be honest.
Just in case... If you want, you can use this script, which makes pressing F button whip your stuff around, you don't need to press R button after F anymore!.
There's a catch though.
If you use inspect (F) while equipped with any gun which isn't full (as in reloaded) it will auto-reload.
You can't really do much about this, but it's not that much of a problem I think.
Here's the script:
alias +igra1 "-lookatweapon; +reload"
alias -igra1 "+lookatweapon; -reload"
Just paste it into your autoexec and boom, done.
Oh, you don't know what autoexec is? Google it! Cheers:)