Looking at the video again reminded me how often Smith, Sonia Sanchez, Amiri Baraka, Tyehimba Jess, and various other poets close their eyes when they are reading. The poets I have in mind do not close their eyes simply to concentrate on remembering particular lines of their poems. Instead, they close their eyes as a way of channeling and displaying intense emotions and distinct sounds.
When Baraka and Sanchez close their eyes, they are likely emulating jazz artists or other musicians who might go through a number of intense facial expressions as they seek to achieve distinct sounds. Smith and Jess, also draw on the visual histories of black musical performers, and given their experiences as Chicago slam poets, they likely also draw on stage performers and the conventions of spoken word--where facial expressions are as much a text as words on paper.
When some of these poets tightly close their eyes during intense segments of their poems, they give off a sense of the difficulty of what they are doing. The strained, pained looks give them and their performances more credibility among audiences and projects the idea that the performance or reading is a serious affair.