Please have a look at the following site. You will find "すり割部押し加工つぼめ" mentioned in the "特殊仕様" of the last example of 高機能加工事例 along with the picture.

窄める, つぼめる, (v) means to make narrower / to shut (an umbrella) / to shrug (one's shoulders) / to purse (one's lips).

押し加工 can be translated as push processing. However, I could not find such technology related to metal processing industry. I think 押し出し加工 (Extrusion processing) has been abridged here as 押し加工. Extrusion processing is a very popular process used to create objects of a fixed cross-sectional profile.

Considering all these, I think すり割部押し加工つぼめ can be translated as "Extrusion processing used to narrow the slits". If you prefer to use Push processing instead of Extrusion processing, you can also choose "Push processing used to narrow the slits"