The meat here is named sujinuki and is fatty yet very flavourful. Suji means tendon muscle and nuki means removing.
Mr.T said, It is just like he showed you at his restaurant that he removed tendon out of otoro.

This how to remove suji:https://tsukijistyle.co.jp/?action=DocInc&doc_type=print&page=rpf_1200-53_00