The Spinners are the Chimera's new way of infecting human hosts and creating new Chimeran soldiers in response to the aftermath of Operation Overstrike, in which the Carriers were compromised and exterminated from Dr. Claude Bouchard's serum, and lessening the Chimera's need of Conversion Centers. The Spinners are released from Spires, just like Crawlers, and move on to capture humans and spin them into cocoons-hence the name. From here, hosts are converted into whatever strain is in short supply and/or is needed.
It is never particularly clear how such small creatures can overpower and restrain able-bodied adults; they are treated as being as unstoppable as Crawlers, but do not seem to be deployed in overwhelming numbers. Indeed, at one point Henry Stillman describes Spinners somehow overturning a moving vehicle, something it does not seem the creatures would be capable of. How Spinners actually "work" remains a mystery.
In Resistance: The Gathering Storm, it is shown that Spires used to carry Spinners contain yellowish softball-sized eggs that hatch into thousands of infant Spinners that are stated to 'morph' into cat-sized individuals. When they find an individual human, they will spray a pinkish brown webbing on them. This chrysalis rapidly hardens after which the Spinners will carry it away. It is mentioned that Spinners are apparently capable of stinging, and that they are equipped with fangs and hollow barbs through which chemicals can be injected into victims, resulting in uncontrollable thrashing. This chemical could possibly be used in the process of turning a human (and other organisms) into new Chimera, such as Grims or Hybrids.
It has also been stated in Resistance: A Hole in the Sky that Spinners need to bite their victim in order to incapacitate them prior to spinning a cocoon around them.