Question about pass-through serio

Hi everyone,

I am trying to understand how serio works in Linux. When reading
drivers/input/mouse/synaptics.c I found a special serio type:
SERIO_PS_PSTHRU. I git-grep the source tree and it seems synaptics
driver is the only user of the type. I cannot find any detailed docs
explaining why the synaptics driver needs this, and what's the
difference between this pass-through serio and psmouse->ps2dev.serio.

Can anyone help me with this? Any pointer or keyword is very much
appreciated. Thank you very much.

jj