Bed - Difference between slats and sprung slats?

[Feek]

As I've mentioned before, we're getting a new bed frame. The one I originally ordered is out of stock with no due date for it to be in stock.

I'm looking at another which is around 50% more expensive than the first choice but it's listed as having 'sprung slatted base' as opposed to the original one which just says 'with a slatted base'.

So what's the difference? Obviously one has sprung slats and the other doesn't, but surely the slat is just the wooden cross part that the mattress sits on. Effectively a plank! How can a plank be sprung?

Answers on a headboard to Feek, c/o The Bedroom.