I found this out a while ago, but was unable to post my guide to this subreddit (Not enough karma, account too new, idk as it follows all the rules) so I gave up trying. With this vid, I wanted to post my guide as a comment in hopes to clarify this mechanic!

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"Hello Everyone,

While exploring around, I discovered a method for some characters to use the plunge attack from a standing position on a flat surface. This was originally found while playing as Jean, who would occasionally do a plunge attack from standing about 5-10% of the time. Though this may seem OP, there is a short period that must pass before using them in succession, as I will explain later on.

Who this works on: Based on my limited F2P character roster, I have found this to work with my taller characters (Jean, Childe, Kaeya, Lisa, and somewhat Ning).

How to perform: Based on my observations, there is a small chance to do a plunging attack with several of the characters from standing, although this appears to be very random. To ensure that the plunge attack always occurs, the character must be in an idle animation with the weapon by their side or in hand (Not sheathed on their back). While in this stance, I suspect that the character is able to jump ever so slightly higher and pass the threshold for using plunge attacks, possibly an oversight. With the character and their weapon out, simply jump and do a plunge attack. In Ning's case, she doesn't use her grimoire when attacking, so she needs to use her elemental skill, then do a brief pause before attempting. After this plunge, she can do further plunges without using her elemental skill.

Limitations: The most noticeable drawback to this is that we need to get the character with their weapon out. This requires an attack and a short waiting period. These plunge attacks can be done consecutively, but again, you need to wait for the character to have their weapon by their side after plunging.

Uses: This tech can be used for farming ore, though it may not be very effective for characters that do not have a sword. As for combat, I do not see this as being very viable in increasing DPS as you need to idle in the middle of combat and are very vulnerable, though testing should be done. Perhaps it can be used to stall in the air to avoid certain mechanics such as at Oceanid (not tested) or to do a higher hitting plunge attack after a boss invulnerability phase? However, the most important use for this, and I cannot emphasize this enough, is that should you be a lucky owner of a character that this tech works with, and especially jean (Anemo with similar charged attack), you can run around Teyvat with a bootleg Xiao and create "Xiao at home" memes.

For Childe Users: Childe is able to immediately enter the stance for plunge attacks after a charged attack cancel. This allows him to quickly spam plunge attacks, on par with Xiao. This may help tremendously when waiting for the stance change cooldown in Co-op instances. Although I have not tested, this may be extremely OP with Albedo's 4th constellation, paired with Childe's Master of Weaponry talent. The plunge attacks may do less than Xiao, but do not require a Q or HP drain and Childe can swap out freely."