It's pretty obvious!

Protein effects how much structure your hair has. When it's really soft and stretchy and undefined, you need protein. As you use protein you ideally want to achieve hair that is "perky" and hydrated. Once you cross into the crunchy dry zone you've gone to far. If your hair was defined to begin with, skip the protein treatments and monitor the protein in your daily products.

It's a pretty immediate crunchy effect if you overdo the protein. Unless you persist with the protein, the damage is reversible with a moisturizing treatment.