Best ways to teach compression depth without a feedback device

Feedback devices are great when you have them, but not every class setup includes them and not every student is going to have access to one. Teaching proper compression depth- at least two inches for adults without real-time feedback can be a real challenge.

What techniques do you use? Do you use verbal cues, physical demonstrations, or creative analogies to help students understand what two inches actually feels like? Share your best methods here.