From Minecraft Parkour Wiki
  • The player's bounding box (0.6 × 1.8 × 0.6) is used to detect collisions. (Reminder: the collision order is Y-X-Z)
  • When a wall is detected while the player is on ground, the game attempts to make them "hop" over it. (as shown in the video)
  • In the end, the game chooses the method that yields the longest horizontal distance.