Sponge hardness is a topic that is often misunderstood. Various amounts of spin & speed are produced on the ball depending on several factors...
These factors are
1)Dwell time (mostly related to spin but in soft rubbers it creates speed also)
2)Power of the stroke (related to spin & spin also, the harder the hit the faster the ball, the harder the brush the spinnier the ball)
3)Tehcnique timing & angle of the stroke that also produce different amounts speed & spin
Some physics to understand better
http://z.about.com/d/tabletennis/1/0...ndiffspeed.gif
There are two forces on the ball a) the tangential one that creates spin b)and the lateral one that creates speed
Notice that by the term speed we mean horizontal speed not vertical! The actual result of these two forces is that the ball is travelling spinny and speedy creating an arc.
Lets assume that a table tennis sponge is like a spring . Hard springs when compared to softer ones
when fully compressed produce more force thus more acceleration (F= K X A , Newton's law) thus more speed, to the object that compresses them. K is the modulus of elasticity of the spring=sponge, F the Force , A fore acceleration
Conclusion no1 : When the sponge is hard you need strong hitters to manage to compress the sponge at its full in order to take full advantage of its abilities ! To achieve that you need a hard hitter with perfect timing and angle of the racket. Like any chinese professional player for example...doesnt have to be top10. That doesnt mean that if you are in good physical condition with fast footwork and good body & wrist rotation (but not a pro player) you cant play effectively with a chinese rubber
.
Conclusion no2 : With softer sponges (the sponge has more "porosity") you need less power and have big margin for error in your angle of your strokes to achieve a desired arc (speed&spin). Because of the softness, the ball compresses easily the sponge at its full potential giving you & me and the whooole TTdaily forum amateur players an easy task!
Conclusion no3 : Knowing the importance of timing, paddle angle and sponge hardness, underspins lifted with hard rubbers need perfect technique and timing compared to the soft ones. A heavy underspin if lifted with a small error in technique using a soft rubber, maybe will not be lifted (or lifted with less pace & arc) with a hard rubber.
BUT when you learn your technique correctly and learn to lift every underspin ball with ease, using hard rubbers will give you more effective shots
So...If you have good technique footwork and physique you can play with a hard tacky rubber like the hurricane (boosted of course...chinese rubbers were invented to be boosted) . If you are a beginner who still is improving his shots prefer a soft rubber.
p.s. Notice that conclusion1&2&3 are considering the exact same stroke with the exact same incoming ball to your paddle.