On forehand you should use harder sponges in order tot involve the entire body to get the maximum power from the hard sponge. Backhand stroke is however more compact than the forehand stroke, unless you are a sort of Kalinikos Kreanga and you can hit you backhand almost as your forehand. So, in order to create maximum power with a compact stroke, you need a softer sponge than on your forehand, so you don't need to use all of your body to develop a powerful backhand.
On pro level they are phisically and technically very strong, but they still use this principle. There are some players such as Liang Jingkun that consider that the powerfull H3 National Blue Sponge is softer than the orange one, so they play with it on backhand.
But still, I repeat, they are pros, they know how to coordinate their feet and arms to obtain maximum power both on forehand and backhand.
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