The best statement your teacher would likely use in the review about signing numbers 60-100 is:
You add the rock/twist to help people see which two numbers you are signing to make your two-digit number.
This statement clearly conveys that the rock/twist motion is used to improve clarity when signing two-digit numbers, which is helpful for the understanding of those specific numbers. The other options might contain useful information, but they are more specific and may not apply universally to all numbers in the 60-100 range.