I am not expert on how to hit it but I saw him play live in a small arena. It was a tour of seniors with Laver, Rosewall, Drysdale, McMillian, Stewart and a couple of others.
Rosewall's bachhand did not have as much spin (slice) as most pros hit today. It was more of a flat drive with a hint of slice.
Most of the courts during Rosewall's and Laver's prime were fast. All the slams were grass except the French.
I would think Rosewall's BH hit hard with little spin would have a very low bounce on the grass courts.
He also hit it surprisingly hard. He could drive a passing shot by a net rusher with it.