I guess this weakness has a great potential to be misinterpreted. I'm even not sure I'm getting it correctly. How I read it:
During each Carson's turn of each investigation phase he has to commit a card to an action of another investigator. It means that this card can only be committed to an action that was granted to another investigator by Carson himself. Well, some other scenarios should also work (e.g. Carson takes Liquid Courage with Versatile and heals his teammate), but they are less likely to happen. Doesn't help to commit a card during this investigation phase when it was a turn of another investigator.