The Juji Gatame or Straight arm bar from guard is one of the most basic submissions in Jiu-Jitsu. However, to catch it in live sparring it’s often an impossibility as it is one of the techniques that is really telegraphed when you know what to look out for.
In this video Nic Gregoriades shows the finer details of locking the submission in. Hope this helps.
Train hard, tap fast and have grips like a sloth.