绷 has multiple pronunciations: bēng (stretch tight), běng (to crack/burst), and bèng (to sew loosely). The most common usage is bēng meaning to tighten or to keep a straight face.
mistakes
绷着脸 (bēng zhe liǎn) means to keep a straight/stern face, not to stretch one's face. Don't confuse with 崩 (bēng, to collapse).