Set two spinning tops on a table. If they spin in the same direction, their combined spin is large; if they spin toward each other, the total spin can be zero even though each top spins rapidly. Neutron stars approaching a merger have a similar property: their own rotation, or spin. The gravitational waves emitted by the pair depend mainly on the average total spin. But what if you change the spin of each star without changing the sum? It turns out that the ejected mass and the brightness of the flash after the merger change by tens of times.
The scientists ran computer simulations of mergers with different masses and spins, but with the same total spin. The mass of ejected material varied by a factor of 45. The yield of elements heavier than barium varied by more than a factor of 100. The kilonova brightness differed by almost one magnitude, and its color by one and a half magnitudes. It's as if two factories with engines of the same total power produced vastly different amounts of product. This means that from the light of the flash, we can determine the spin of each star individually, something that cannot be done from gravitational waves.
In one model with anti-aligned spins, fast ejecta (with speeds exceeding 0.6 times the speed of light) reach a mass of about one thousandth of a solar mass — tens of times more than in typical non-spinning mergers. Such ejecta could be the source of the fastest components of the kilonova.
In Arthur C. Clarke's novel "Rendezvous with Rama," an alien spacecraft uses a gravitational slingshot around the Sun to change its trajectory. Neutron star mergers are a natural version of such a maneuver: the spins of the stars, like invisible gyroscopes, steer the outflows and determine the fate of the system.