I was reading the patch notes and found this:
BUFFERING Katarina can now buffer Hextech Gunblade's active to fire after Shunpo's cast time completes.
What does buffering do and now do you enable it? From the sounds of it, buffering makes items instant cast after a spell it is linked is cast. How do you enable this? Is it just passive...?
[sg-lux-2]
iPooUnicorns1 votesIt's another word for queuing spells, where you give additional commands before the original command has been completed.
The Dreamwalker1 votesHow does someone go about doing this though? Is it entirely passive? Does it only work for targeted spells or do skillshots work too? Or are there button set up things?
Modl Ryden l1 votes[{quoted}](name=The Dreamwalker,realm=NA,application-id=3ErqAdtq,discussion-id=OqvIREeQ,comment-id=00000000,timestamp=2019-01-14T20:06:48.231+0000)
How does someone go about doing this though? Is it entirely passive? Does it only work for targeted spells or do skillshots work too? Or are there button set up things?
No fancy setups needed, you just need to make sure of a couple things
Some things to note when performing buffer, you pressing the skills while skill you're currently in is Channeling.
If you see an ability channeling for your champion, you're most likely allowed to buffer an ability or two before you finish channeling.
So step-by step in the case you mention with Katarina's Shunpo and Gunblade:
If you want to practice this in the Practice Tool do these steps:
Have two enemy champion dummies. One to Shunpo to (we'll call dummy1) and one to cast Gunblade active on (dummy2). This way you can see the FX of Gunblade's active to confirm you are buffering correctly
Have cooldowns on auto refresh
But if you want to practice on other champs, Irelia and Ezreal have huge buffer windows on their resepectives Qs
If you go to the Practice Tool with Ezreal and maximize CDR (build 45% from runes and items)
MagicFlyingLlama1 votesBuffering is when you use an ability or spell on someone out of range, which causes you to move into range and automatically use it.
It also applies to the same abilities when you try casting them during other actions or CC, which is what this refers to. You can usually queue up to one additional action.
Vesarixx1 votesIt's when you issue a command for one action while another is still in progress, for example, Alistar can buffer his combo by using pulverize during the animation for headbutt and pulverize will activate once headbutt completes.
It can also refer to using an ability to mitigate the effects of CC, like if a Nautilus is about to be hit by a morgana Q he can cast his own Q before it hits and if timed right he will still be drag himself towards whatever the Q hits.
The Dreamwalker1 votesSo, is it entirely passive then? You don't need to set up any special button prompts? Is this like 
Q - R combo sort of thing?
Vesarixx1 votesThe only thing you might want to do for best results is enable quick cast, other than that it's as simple as pushing the button for one ability while another is still casting, it doesn't work with every ability though, so you might want to try it out in the practice tool with the champs you play before using it in game. You can't queue things with channeled abilities for example, like if you cast 
R and then tried to use Q it would cancel the R instead of waiting for the R to finish channeling and then casting Q, I'm not actually familiar with a Q+R combo on MF so I can't really say if it uses buffering or not.
There are a few different ways that abilities can interact Ahri can use buffering by pressing E and then immediately flashing, the E will start at Ahri's new location after the flash and still go towards where your cursor was before the flash You can flash during Ezreal R and the R will leave from Ezreals original position before he flashed, instead of following him If you cast W during Fiora's Q animation the W will actually go off during the Q, and doesn't need to wait for the Q to finish casting(this can help you go through a veigar cage and not get stunned btw)
Another way you can use this is with abilities that need a target, if you are out of range to use an ability and you try to use the ability on an enemy it will attempt to put you in range to use the ability, like if you were playing Vayne and you thought a champion was going to flash on you, you could press E while they are still out of range and the ability will apply as soon as they get in range, this will cause you to walk towards them in an attempt to put you in range, but you can cancel it by just moving somewhere else.
The Dreamwalker1 votesOh okay! Thank for you the detailed explanation and suggestions. So, that is why run towards the enemy while they are out of range... that also explains why my Q randomly fires after I click away from the enemy but then they jump on me. It also explains why sometimes my Q doesn't fire after I click on R on Miss Fortune. Do auto attacks override these queued spells or no?
Also, one final question (I believe I know the answer), but quick cast is the act of just clicking (for example) Q and the ability just fires without an indicator. Is that correct? How would I go about enabling quick cast? I think that would be beneficial to use on .
(mainly talking about MF because she is literally my main character. As is Lux.
Vesarixx1 votesyes, an auto attack would cancel any spells that were queued up as long as it is input before the ability starts or it is an ability that can be interrupted with an auto attack.
might be worth mentioning that you cannot buffer anything after this items active, like if you cast 
ult out of range and then flash into range the ability will go off on its own, but if you use protobelt to get in range it doesn't cast whether you press R and then protobelt or press R when the protobelt active is happening. I think Riot did this on purpose because they intended the item as a defensive tool.
Liluzifangirl1 votesis there anyway to turn it off