The kettlebell armbar is a fairly advanced mobility exercise, used to improve thoracic spine mobility whilst also providing a stretch in the Pectoralis Minor.
It's really important to stress that you should only use a light weight for the kettlebell armbar. It's a mobility exercise and the function of the weight is to provide proprioceptive feedback, NOT to enhance strength.
You would also be wise to have someone spot you the first few times you do this. Sometimes, our proprioception is a little off and that safety net is valuable to make sure you don't hurt yourself as you learn.