The Platboom is pretty simple — you can use it like a platform that can also squash enemies. Think of it as an elevator — it rises up before dropping down and crushing enemies.
It costs 3 energy (▲) to summon, so fairly expensive in the early hours, but when Tri reaches level 8, its cost will drop to 2 energy, meaning you'll eventually be able to summon three Platbooms at once.
Platbooms might just be one of the best Echoes in the game as it can get much higher than multiple Beds or Water Blocks at an overall lower cost.