She waits a few seconds, watching him, waiting for him to turn and spot her, but he's so focused on what could be in the other direction that she has to seize the opportunity. She's a little afraid that there might be something bigger and stronger that Sam's spotted in the direction he's looking in, that would take both of them out, but she creeps toward him nonetheless, light on her toes (put a syrio quote here). When she's close enough she leaps, attaching himself to his back and raising her arm to strike with the sword.