There is also besides the height of the take back something else. The modern guys wait a split second longer to initiate the loop. To preserve the continuous loop. From unit turn to contact. Older era might wait with the racket behind them more often. I'm not even sure if the term unit turn is appropriate for the older era closed stance fhs.
Thanks this explains it really well. When I watch Wawrinkas backhand takeback I see him coil very early. Then he appears to stay coiled for a split second before he unloads. Same for forehands of all the top guys. I see them turn and coil very early, then they have their racket head high for a split second before it drops down and starts the "loop".