Working Hard
The first spell in the diary was the one about gravity. Changing a specific area was the core of that element, so the author of that diary wanted to make sure the student would practice it from the start.
Kai already knew the spell — he could change the gravity of a small area, and the change wasn't that dramatic, too — but it was a start.
Alastor didn't know that, and if Kai showed up doing the spell first try, it would raise suspicions. He wasn't too concerned about it because of the loop, but it was still a good thing to act normal.
So, he read the spell for Alastor, and then started practicing it on him. It required a specific set of glyphs, and the concentration to make the spell work was tremendous.
Kai tried for at least 20 times before doing a successful attempt, increasing the gravity around Alastor, thus making his body way heavier to move.