Be careful out there!

My next door neighbor was a master mechanic. He agreed to teach me during my rookie days. He emphasized safety and gave me some tips. To this day I still use wheel chocks and a pair of roller tires mounted on steel rims that go under the car once I get it high enough to crawl under it or remove the wheels.
I'm trying to imagine what your "roller tires mounted on steel rims" actually looks like? Could you post a picture? I'm intrigued.