To achieve a submission in Brazilian Jiu-Jitsu, you generally need to isolate the limb of an opponent (such as the arms, legs, or neck) from their body so that they can't can't bring their body to bear to escape or counter the submission. Usually this means you are either putting some part of your body between their limb and their body or otherwise stretching their limb away from their body so that they can't bring it back close. This leads to effective setups of submission techniques.