From the author's README on Thunderstore

Casper Slide Plug-In

This is a plugin to implement a casper slide.

##Notice: This plugin is "vibe-coded" and does not replicate a "true" casper slide cleanly or accurately.

How it works:

This is a visual Casper pose only. No animations are used: the plugin flips the board upside down and tilts/rotates it to mimic the Casper position. It does not implement physical sliding, dark-catch or manual detection.

##Controls: By default L3 + R3 (press both analogue sticks IN), however there are other preset binds.

##Console Commands: casper status casper on | off casper bind 1-10 casper direction -1|1

Credits:

Credit to bot who helped me figure out how this works

Install Casper Slide

You need ReSkate and your own copy of skate. on Steam.

Open ReSkateLauncher.exe and go to MODS.
Search for Casper Slide and click install. The launcher also installs anything it depends on.
Press PLAY. In game, the MODS tab of the ReSkate menu (Insert) turns it on and off.

Or install it by hand: download the .zip and drop it on the launcher window, or unzip it into its own folder in Mods\ beside Skate.exe.

Maps show up in the ReSkate menu's level list. To join a server running a custom map, you need the same map installed. If the game won't load after installing, see known issues.

skatemods doesn't host or review this mod. The download comes straight from Thunderstore. Only install mods you trust, because scripts can run code.