I am Posta Longa (Long guard) and I look like this. And in the hold you want to do to me, I will surely bring my right arm under your left arm. And I will enter the first play of grappling, and with that hold, I will make you go to the ground. And if that hold is likely to fail me, I will transition to other plays which follow.
This is a waiting guard providing a good invitation. From here you can enter into the first play of grappling.
Be sure to keep the forward arm on a diagonal across the body, covering the left side, so as not to leave open possibilities for attack from the player. The left arm remains relaxed covering the lower quarter.
Like all posta, the arm and body should not be rigidly held in the same position and you can adopt variations as needed. You can, and should, be comfortable changing sides so as to open different lines.