Steps to recreate:
1. Go to any world with cheats enabled

2. Put this command in a repeating command block: clear @a[nbt={FallFlying:1b}

] minecraft:elytra

All this does is clear the player of their elytra if they use it, which should put them into a freefall.
3. Pick a spot, build 3 blocks up, and create a horizontal row of blocks

4. Equip an elytra

5. Jump towards the row of blocks from the ground and time your second jump to enter elytra mode as you start falling while holding forward (this may take a couple of tries)

I've linked a video of the setup and glitch if that's helpful.