Wooden Posts are objects appearing in Bob-omb Battlefield and Tiny-Huge Island. They can be ran around to release coins, and one post is attached to a Chain Chomp who is released when the post is ground-pounded 3 times.
When Mario ground-pounds a post, if it is not already being ground-pounded, a sound plays and the wooden post's Y-speed is set to -70. When the post is ground-pounded, its Y-speed increases by 25 every frame until it reaches 0. In practice, this means that the post drops by 45 units on the first frame and 20 units on the second and then stops moving. If the Wooden Post's Y-speed will put its Y-offset below -190 units, the post's Y-offset is set to -190 units and if a chain chomp is attached to the post, it is released.
When Mario gets within 400 units of a post, a timer begins counting up. The game sets a variable for Mario's total angle to the post and each frame this is incremented by the value of Mario's current angle to the post minus his previous angle to the post. When this total angle reaches a value greater than 196,608 angular units (3 full angles), if the timer set is less than 200, 5 coins are spawned from the post. In practice this means that Mario must run around the post 3 times within 200 frames to get the coins. However, with TAS precision, Mario can achieve the total angle value needed while above the post in the air.
Eru Post is a method to obtain coins from a Wooden Post without running around the post. The method works by having Mario jitter around in the air above the center of the post in a star pattern. By doing this, the game concludes that Mario has circled around the post several times and spawns the post's coins. This allows the coins to be spawned after a single jump kick or dive recover, making it much faster than the usual method.
It is also possible to do a variation of Eru Post by low-magnitude walking on the post.