Yeah it didn't make much sense to me either which is why I initially asked the other poster about what he knows about the process. From Googling, it looks like the tension is provided by a chemical treatment process.That doesn't really make sense though, you need something rigid to hold the tension without deforming. Sponge is elastic, it wouldn't be able to stay flat if it's under tension (topsheet would apply tension back onto the sponge, equal but opposite reaction).
I guess that would mean that if you can't stretch a rubber mechanically on its edges, you can stretch it somehow internally by mixing other compounds in. Would be interesting if there was more explanation written about tensor 'technology' rather than the endless marketing we do get.