When Vorden explained to Leo about his abilities, Leo was doing his regular thing checking to see if he was speaking the truth or not. It was for this reason that Leo wasn't as cautious as usual, because he saw that Vorden wasn't lying.
From experience, Leo knew it wasn't a good idea to have contact with someone's ability you didn't know or trust. Certain abilities had conditions that needed to be met before using them. This was called a cost.
Cost abilities were usually stronger than regular abilities but there were only a few that the public could obtain, most were kept secret by the originals. Two examples of cost abilities were Vorden's where he needed to touch a user's bare skin before being able to use their power, and Jacks who could only absorb their power through the user's mouth.
