All insurance markets have different stances on the open pilot warranty. Depending on the company/aircraft/policy, your open pilot warranty will allow certain individuals to fly your aircraft and be covered under the insurance policy. For example, if it lists someone with a Private or better license, 500 total hours and 25 make and model hours, that person (with your permission) will be covered under the policy should something happen. But, if someone is flying your aircraft that does not meet the requirements- proper ratings, hours and make and model time- if a claim occurs, you will be liable for any and all costs.