Crescendo means to get louder and Diminuendo means to get softer.
When you want to play a Crescendo Tambourine Roll start by playing your tambourine way down low and as soft as you can play. As you roll louder, raise the tambourine up high as you shake it more vigorously.
When you want to play a Diminuendo, perform the crescendo technique in reverse. Start way up high playing loudly and vigorously then lower the instrument as your roll gets quieter and quieter.
In the video I’m playing this technique without anything like a music stand or other musicians in front of me but when you’re in the band or orchestra and there are things like people and equipment in front of you the effect of your crescendo and diminuendo roll is enhanced.