Learn Swedish: På det här underlaget är det själva smidigheten som räddar dig. - On this surface, it is the actual agility that saves you.

It acts as an emphatic determiner meaning "the actual" or "[the noun] itself". When used this way, själva is placed directly in front of the definite noun (smidigheten). Notice that you do not use the freestanding definite article (den) before själva, even though the noun is definite.

Why is the sentence built around the structure är det ... som instead of just saying själva smidigheten räddar dig?

This is a "cleft sentence" construction, which is used to put heavy emphasis on a specific part of the statement. By breaking it up into "it is [X] that [Y]", the speaker strongly highlights that the actual agility is the exact thing saving you, in contrast to whatever else might normally save you.

What exactly does underlaget refer to here?

The word ett underlag literally means "an underlying layer" or "base". In the context of sports, driving, or walking, it refers to the physical surface or ground conditions, such as ice, gravel, or a running track. In business contexts, it can also refer to a figurative foundation, like data or documents used to make a decision.

Why is the word order är det rather than det är?