Credits to Alanim.
Mack Skip consists of three jumps with very precise platforming.
This is one of the "gatekeeper" techs of the run that you will need to learn early. If you drop Mack Skip, the run is over. Luckily it's only 15 minutes into a 3 hour run, so try again if you miss it!
How it looks in action
Quick tutorial - Caerulius
A quick tutorial that explains dead frames and shows the proper Mack Skip setup. Turn on annotations.
Detailed tutorial - Justin-credible
This video is the most recent and goes in-depth explaining why Mack Skip works the way it does and addresses troubleshooting.
Detailed tutorial - Alanim
Alanim discovered the current Mack Skip setup. He explains it thoroughly here: https://www.twitch.tv/alanim/v/63608618
Troubleshooting tutorial - pidgezero_one
If you've watched all the tutorials and still can't seem to get it down, this video might help you see exactly what you might be doing wrong either with your setup or your inputs.
Here is a better explanation of Pidge's visual cue for the second jump. This may or may not click with you, but if you're having issues with dead frames, give it a shot.
Images & written guide
How to do the first jump: Your first jump will be off the ground onto the upper level behind the Shy Guy.
It's important not to do the first jump too soon, or else you will misposition yourself and start the cutscene right away.
While it's rare, there may be times where you do a running jump off the floor and right onto the Shy Guy's head while he is landed. This will trigger the cutscene unless you have inhuman reaction time. It's unfortunate when this happens but is amusing nonetheless.
How to do the second jump: Your second jump will be off the upper level onto the Shy Guy's head. This jump is tricky. You need to be holding right and will move over 5-6 px or so while in the air.
- If you are standing after landing on his head, you did it correctly.
- If you land on his head but Mario is forced to walk off and initiate the cutscene, that usually means you did not get enough horizontal distance in your second jump. It's easier to do if you're holding Y.
- If you land on his head but fall off without a jumping sound, that means you held the D pad too long. Release it before landing.
- If you fall through him without touching his head at all, you landed on a "Dead frame". 5 out of the ~29 frames in the Shyster animation will force you to fail Mack Skip regardless of how your first jump went.
- This GIF by pidge shows where the dead frames are. Do not press B on any frame marked with a white X. The dead frames are mostly concentrated around the apex of the Shy Guy's jump, so you are safest jumping while he is mid-air after he begins his descent down.
- Some runners like to secure their position by releasing and re-pressing Y to move 1 pixel to the right before initiating this jump. This is not necessary to do, but may help you with positioning.
How to do the third jump: Your third jump is off the Shy Guy's head over into the corner where Chancellor is.
There is an invisible wall beside the Shy Guy. If you do not jump over it, you will slide down and initiate the cutscene if you don't react quickly enough to get back into position.
It is safest to initiate this jump when the Shy Guy is at the peak of his jump. It is recommended to move down and left on the shy guy's head (the platform of his head is diagonal-shaped like a / ) so that you will be at the peak of your jump height when you pass the invisible wall. Be very careful when moving on his head as you may fall off if you go too far down.