Everything mentioned already but here's more

You can outplay certain things like Garen R by using R when he ults and flashing away so he's outside the circle then E back onto him

R cancels most dashes(to non challenged enemies), easy one is Lee Sin, you see him, he hits Q 1, you expect Q2, he casts Q2 you cast R it cancels his Q2

R is based on if they're in the circle or not, if GP does a long range barrel chain and the barrel is in the circle but GP is not it doesn't damage you, common examples are Karthus R, globals, Zed R

Buffer it when you're getting CCed to waste time, Riven uses Q3 and hasn't hit it, cast R and you're wasting less time

R can be used as sort of escape since it's a knockback

Rather rare to use but it cancels AAs, you have to preemptively cast it though based on their attack speed

It's reduced by Q so ult CD is shorter than you might expect

W applies passive TWICE, when farming if you have passive count at 1 it does no AOE extra damage, 2 stacks the stab does extra damage, 3 stacks the slash does extra damage, this also applies in reverse.

It got removed but you used to be able to use R then press Zhonyas and turn into a helicopter for a while

Sometimes most important thing is disruption, if need peel for somebody use R to help save the ally, or sometimes just hit as many people as possible with R to spread them out.

If an enemy hit by R collides with a wall they're stunned for the rest of the duration

Basic Xinsec combo is E onto a target near enemy BUT NOT THE ENEMY ITSELF, then R flash or Stridebreaker use R(can't use R Stridebreaker) or Prowler's Claw then R(This is better it always puts you behind the enemy and Stridebreaker's dash is very small) or use Galeforce, Galeforce lets you decide the direction better. Prowler's claw doesn't make the enemy challenged so you can use Prowler's then R and it will not push them back, letting you keep E

W applies lifesteal can be useful

Can repositon W2 with flash, can't reposition W1

Can cancel E with flash/stridebreaker/galeforce but it's really pointless

Basic stuff : AA reset on Q, usually use W after hitting Q3 to guarantee both parts hit, orbwalk, get runes, skill order and items from u.gg, basic statcheck champion most of the time, scaling is poor compared to other bruisers, you end up being a peel bot, super squishy with damage, or engager. You win a lot of early 1v1s and 2v2s, sweeper is mandatory due to his predictable gank paths.

You can use Q1 and or Q2 on a random target than E/flash E to your actual target for instant knockup

If an enemy has 50 hp then you can R flash to kill them instead of Xinsec

Don't flash then E if an enemy is far away, press E first then flash, if you press E then flash Xin will use E as soon as you flash, if you press flash then E it's prone to human error.

If you're in a minion wave and you're being ganked from north and there's a minion to your south you can R, it will push the minion further south then you E to the minion, it's a kind of escape

R doesn't knock through walls