rogerch here is Christian's explanation:
My understanding that it's similar to grace notes. In the comments, somebody asked him the difference he says:
Grace note comes from classical, is very controlled, preferrably played with two fingers and supposed to sound gracious. Slide: preferrably with 1 one finger to create a smeered sound. Control is neglected in order to sound dirty.
hebele I see. This is the reason I didn't find the slide in his video, because I was curious about Christians finger positions. (Not that I would change mine, if his would be different. Nope - I even change his fingering numbers too often.)
But it is slideable. It was instantly in my muscle memory to be recalled. Therefore, I guess he did the royal thirds with that slide somewhat later. Probably around lesson 10 or later. In lesson 2 definitely I didn't play this slide, because I only followed the video.
Maybe you should skip this specific slide for now and go on. If it was the comfy slide without the need for the curled fingers, I would encourage you to do it. Don't worry - you will learn it later, and you are missing nothing for now.
Somewhere in this thread I was complaining about the slide too (yeah yeah cue WieWaldi "you moan a lot" pffttt) and I think I even mentioned it hurt a little. But... I learned it too. And thanks for the video about the slide @hebele - I missed that one too! Interesting!
Edit: Found it! In fact that was the exact same slide (without the C). And WieWie posted a video then too 
Sophia (yeah yeah cue WieWaldi "you moan a lot" pffttt)
No, this is not true. I am not moaning a lot. I am just complaining that my progress is so slow. And I don't like, that I forget more than I learn. And I am jealous that you are learning so much faster than I do. And I am envy, you learn all the advanced stuff in Alfred's like diminished chords while I stick to the Blues stuff only. I mean, Christian only explained the half-diminished chord so far. You play Piano for several years less, but you know already how to diminish something fully. I can only half diminish a chord. And that you can play Chopin, and me not. You know, there is no easy Chopin. Not to mention this Ostinato Jazz. I use Blues as a vehicle to get into Jazz, but you step directly into Jazz playing. And then all this creativity - embedding cats into videos. Doing duets with a hand puppets and all the like. You have two professional musicians as parents giving you all the talent while my parents have trouble to spell music. Actually nobody of my friends play an instrument or is musical at all, and...
...you are right - I moan a lot! 
Edit: 
this is not true!
Wait, no no no noooooooo I didn't say YOU moan a lot. I said you can accuse ME of moaning a lot. Which of course would be totally true. I guess that means we BOTH moan a lot......... we should start a club. Ok, maybe not. I think that might give some wrong impression. 
WieWaldi Actually nobody of my friends play an instrument or is musical at all, and...
You have at least one! Annoying, yes, but talented. Or so she keeps hearing 
Ok, then this is Christian's original video with that slide. Not that different from our beginners versions.
WieWaldi But annoyingly talented
I will agree with the annoying part 
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............ annyong one is me! not you!
Before we misunderstand each other again
Hey @WieWaldi you and the other students of the blues here are way ahead of me on blues playing!
I have done a very small amount of blues playing. My latest was an attempt improvise using the blues scale in the right hand with I IV V chords in the left hand, but using more my style of playing than traditional blues. I quickly realized that to make this work I need to (1) study the blues scale and (2) if I want anything like the a blues progression I need to drill the left hand to make it automatic. Otherwise I quickly lose the blues progression and wander all over the place harmonically.
Basically I'm an absolute beginner (is that right?) when it comes to the blues!
rogerch Basically I'm an absolute beginner (is that right?) when it comes to the blues!
WieWie's scales of proficiency are very fluid and argued over a lot, so you pick your own - all the way up to eternal beginner I am being teased about being at Ferrari speed, but just wait for the sonic jet boom sound once you unleash your powers 
Well, I took another shot at it today with the same result.
I think it would be really cool to be able to play blues improvs and I may do the work to make that happen at some point. Like everything else with piano it will take time! Lots of scales and drilling! I don't anticipate any sonic booms!
I've been working on improvising waltzes for the next PW themed recital. It comes down to doing what I already do, but in 3/4 time rather than my current 4/4ish maybe random time. Even 3/4 time has proven to be much harder than I thought it would be. Blues will be orders of magnitude more work!
Okay, I just recorded, edited and uploaded my recital piece (for the other forum):
I practiced this without the pedal, but today I decided to bring the pedal in. And suddenly my fingering was different. No longer the need for some awkward hand position just in order to hold down some keys for legato playing.
I've learned my lesson: start with the pedal right away, no matter if you think the piece is so easy that it doesn't need the pedal.
Unfortunately, at the very end, one note was played so silent that it is almost muted. This muted note is a result of the changed fingering. Nah - nobody can expect a total beginner to play flawlessly.
Oh that was absolutely LOVELY WieWaldi... I thoroughly enjoyed it and played it again right away. And then again. Then my husband came in and asked what are you doing? So I told him I was listening to your piece... and right now we're BOTH listening at high volume. And I will play it quite a few more times... this is my type of music! Gentle, melodious, pretty harmonies, and very well played. So very smooth and suave too. How dare you call yourself a beginner 
I am really enjoying my current lesson, but now I just can't wait to learn this Country blues too! 
And I agree, the pedal is a necessity... it changes everything from meh to wow!
Sophia Thank you for your kind words. But you know why we call ourselves beginners. The hurdles to become an intermediate player are just too big. You know, sightreading, repetoire of 500++ pieces and you know what else we have forgotten. So much more stuff that is a must have for an intermediate. I guess, I will get 20% of that in the next 10 years.
Unfortunately I am half through the Eight Easy Blues pieces. Only four more to go. 
I wish Christian had 40 pieces with that easy beginner level. So far, every piece was a joy learning and also a joy playing it.
Btw, Christian made this ad-video. Playing a small bit of every piece inside this collection. But this video was unlisted in YouTube. Only very few views, no comments, no upvotes.
I asked him why it is unlisted and he told me... nope - you can read everything in the comment section of the vid. Short story - I am his best promoter!
Friend! 
But annoyingly talented 
