Coruscation

As explained at Pimco:

Put more technically, the value of the Greenspan Put will rise
exponentially if the curve inverts, while the cost of "buying"
that Put will actually become negative: in an inverted curve,
a duration-equal barbell of cash and long bonds yields more than
a bulleted portfolio. Such is the weirdness of an inverted curve:
the less volatile, convex barbell structure actually yields more
than the more volatile, less convex bullet. Rather than paying
for insurance, you get paid for taking it!