The martial skill in his hand was a fire element, and Lin Yi only trained in the wood element as a starter-he had enough Foundation to handle it.
Using wood to start a fire, the fire Qi would be purer and more stable. It would no longer be a single explosive force, but a longer lasting one!
The new fire spirit sword technique had reached the peak of explosiveness, but Lin Yi's incomplete martial arts technique required a relatively stable and long-lasting output, so he needed the wood element to guide it.
Other than that, Lin Yi realized that the basic body strengthening technique wasn't enough to activate the incomplete skill in the book!
Failed!
Failed!
Failed!
After several failed attempts, Lin Yi wasn't discouraged. It was an incomplete skill, and he had to figure out how to use it. It would be weird if he succeeded right away!