Jumping at the same time as dismounting a horse begins the player jump at the top of the horse hitbox, effectively allowing the player to jump 2 blocks.

Pressing jump, dismount, and a directional key while on a horse leads to the horse staying still while the player dismounts the horse and jumps again mid air. When used as such, the player can accrue a jump height of 2 blocks, allowing them to jump out of a fenced in area while keeping the horse inside. Alternatively, this can be used after jumping with the horse to get the player to a height half a block above the horse’s typical jump height. In this way, it is much like a yoshi jump from Super Mario Bros 3. Ultimately, this is a niche bug with few negative results and interesting gameplay applications, but it clearly seems unintended.