Jessica Biel is another one of Bruno's clients. The dumbbells she's holding in this video might not look all that big, but trust us—these sort of Single-Leg Squats (a.k.a. "Pistol Squats") take an insane amount of strength.
If you doubt us, try it yourself.
It looks like Biel's working in sets of eight reps per leg, which is a great range to be in when building muscle is the goal. A problem a lot of strength training newbies run into is performing way too many reps in a set, which is a symptom of using way too light of weight. Biel can barely eek out that eighth rep on her right leg—she finishes the set with maybe one rep in reserve. That's the level of challenge you need to build legit strength and muscle and see serious changes in your body.