There are many reasons students can't participate in rehearsal. Forgotten instrument, broken instrument, sick (but still in class), forgot music and no time to fetch more copies. Etc. That's no excuse to not keep students engaged. This form is for that student. More effective than giving them busy work or telling them to "just follow along," this form keeps students engaged from the start of class to the end.
It tasks them with being rehearsal observers, and reflect on students, instruction, and themselves. I often find the students' insights from this form informing and interesting. Depending on how you grade participation in class, this form can also factor in. In my own classroom, I give students with an unexcused participation reason (forgotten instrument, headache but no note, no music, etc.) as partial credit, and as full credit for students with a documented excused reason (instrument in shop, braces issues properly documented, etc.).
I have found this form incredibly useful in keeping students who can not participate fully engaged and on-task.
What's different that the free version? This one includes a bigger FAQ on how to use the sheet in your instruction, AND includes a Google Form to use instead for digital-leaning classrooms.