Commands are:
? transmit status info
e <byte> echo <byte>
s select
u unselect
a <byte> set A signal to 0/1
b <byte> set B signal to 0/1
x disconnect from SPI bus
80-bf write and read 1-64 bytes
c0-ff write 1-64 bytes
So for example to select, then transfer two bytes 0x12, and unselect, the host sends 5 bytes:
s
0x81
0x12
0x34
u
The command 0x81 is a two byte send/receive, so two bytes are returned to the PC.