This is one of the most growth-promoting biceps exercises you'll ever perform. It requires strict form, which reduces momentum so you'll hit the biceps with continuous tension.
Kneeling Kettlebell Curl, Contracted Position
While kneeling on a bench, curl both of the kettlebells to the top contracted position, about mid-chest height. That's the starting position. Lower one arm and do a curl. Repeat with the other arm.
The key is to hold the non-moving arm in the top contracted position throughout the set while alternating from side to side. Doing this same exercise with dumbbells isn't nearly as effective because the top of the curl involves little tension – you're getting a semi-rest period during the isometric phase.
But because of the unique loading features of the kettlebell, the top position provides constant tension throughout. Also, the kneeling position keeps you from twisting your body to relieve the tension. Any squirming, shifting, or cheating will cause you to lose your balance.
Sets & Reps
Because of the longer time under tension and extended time between reps, 2-3 sets of 5-7 reps per arm will be more than enough. As you get to the end of each set, the pain will be almost unbearable, but the results are worth it.