Ana's weapon is interesting, since it can be fired in 2 ways. Hipfiring it is very fast, however the projectile travels as a slow projectile. Scoping in takes time and takes away some of your mobility and your field of view, but it allows your shot to reach the target immediately, and you don't have to lead the shot either as it is hitscan.
But what about situations where you don't want to lose out on mobility or field of view, but not on the hitscan nature of scoping in either? Quickscoping!
Quickscoping sacrifices some firing rate, however it is perfect for situations where it doesn't matter how much you heal, but IF you are going to heal at all. You don't need super high HPS all the time, sometimes you just want to trade speed for consistency and this is why quickscoping is useful. Quickscoping allows you to fire a shot that is hitscan, without narrowing down your field of view or slowing you down.
So how do you do it? It's actually not that hard with a bit of practice. All you need to do is scope in and as soon as the animation is almost done you fire. I personally don't use toggle Scope so this is how I do it, but it is perfectly doable with a toggled scope as well! Use what you are most comfortable with!
One VERY important thing that a lot of players don't realize is that you need to wait until a new shot is loaded! If you try to quickscope too quick in a row before the gun's bolt has been closed, you will go out of rythm. Always wait until the weapon fully closes before you try to fire another shot, this is super important!
You can also see how I jump with each shot. This is so that I don't lose any speed if i mess up the quickscope and end up in the scoped animation for too long. Its basically something to fall back on but if your scopes are perfect you dont have to worry about this!
Well, that's all you need to know about Ana Quickscoping! Make sure to not overuse this though! This is only really something you should do if you are trying to shoot someone like a Genji for example that is jumping around! Only use it if a little healing would be better than no healing!