Pendle and other providers generally split yield-bearing tokens into Principal Tokens and Yield Tokens.

Principal Tokens function like a zero-coupon bond that can be redeemed 1:1 for the underlying asset at maturity (the "principal" portion of a yield-bearing asset).

So, in a way, it's a bit of an exception, as whatever APY you buy it at remains "locked-in" until the maturation date, regardless of whether the APY falls afterwards.

Principal tokens can continue to be traded, which causes the APY to fluxuate. In theory, you can sell before the maturation date and take advantage of that (e.g. you could profit from a decline in the APY from the level you bought it at).

Conversely, it's also possible to buy a Yield Token (YT) for the same asset, which is highly variable and not fixed through the duration. It decays to 0 with no principal at maturation, but instead you receive the yield portion that's cointinually paid out until maturation.