It’s still a bit with only two possible values. But they add a scaling factor to a group of them (128 for example) which when you factor in, results in a fractional number of bits per parameter.

I believe the scaling comes in later, to turn the 1 and -1 into large numbers that may or may not activate the next layer.

The way they do it is packing like the other comment says.

Each byte represents 5 trinary values instead of 8 binary, and there is a little bit of waste.