In this sketch you can notice a bow-tie or a butterfly shaped object that is placed in the middle of the top's crack.
This object acts as a locking or securing agent, preventing the crack from progressing. The way to make and install it is quite simple:
Make the key; trace its shape on your board; mortise its resting place; bury it in the wood with the help of glue; plane down any remaining protrusion so the key is flush with your board.
To make the key, I started by drawing its shape on a small piece of walnut. I then held the blank in a machinist vise.
I used a saw to cut a few kerfs. Then I pared down the wood until I reached the desired shape. The kerfs make the paring action easier.
Next I took the key and placed it on my board and traced its outline on the wood.