Gas Bags....Fan straps are to lock the fan to the fan OGV's when you need to do just a "core engine" run. I've done it a few times for under core cowl leak checks. As the fan is locked you get no by-pass airflow so can get your head under the cowls for inspections with the engine running.
As for windmilling - there is probably a time limit, wind speed or such like in the AMM which stipulates how long windmilling is allowed for. I'm pretty sure there is limits for in-flight shut down windmilling as well where inspections are required if windmilling was over a certain time.