Practicing in the dark helps us to listen better. We can hear intonation, tone, expression. The practice partner can use a FLASHLIGHT to check one thing at a time: Bow hold, flat left wrist, general posture... It's also a fun motivator for kids to play in the dark with a flashlight!
Thanks to James Hutchins for sharing this and other ideas in his Parents as Partners talk, "Guarantees for More Progress During Practice, the Sequel"! http://suzukiassociation.org/ppo/13356/ for those registered.
I just love that playing the cello is something we can do in power outages - as long as we have some repertoire memorized!