Furniture and Furnishings - For a sofa, what is a "unidirectional suspension"?

Ok...

I have a sofa (American Leather) which claims to have an "advanced unidirectional suspension"-- supposedly it uses some form of webbing for support instead of using springs.

Now I've hear of 8-way hand-tied springs at all... but I just don't understand how webbing on it's own can support a sofa... could someone enlighten me?
It just doesn't sound very durable... but then again, I know very little about how furniture is made. I'm not even sure I know what webbing is... isn't just a tight fabric?

I only ask as my sofa is starting to sag just a little in the back of the seat when you sit down... and I'm wondering if it's something with the suspension or if it's the high-density foam cushions giving out after 5 years of use.

Is there some way to firm up a sofa?

I'd guess "unidirectional suspension" is a fancy name for webbing
It does sound nicer
Webbing has been used for decades
It's often a criss-cross of straps
They could mean a loose knit fabric
There's too many ways to do it to guess which one they used, you'll have to find out
Take off the seat cushions or flip the sofa over and check it out
Post back what you find
If you can take some pics that might be helpful

Basically the sofa should have a wood (or metal) frame
Since that's kinda hard, the seat supports are often springs or webbing
You need to find out if it's the supports, or the cushions compressing