how the heck do ps/2 Y cables work?

Hi,
I was beginning to get a really good feel for the ps/2 mouse
and keyboard protocols, until I discovered a 3 dollar product
which connects both devices to one ps/2 port on your laptop,
using a splitter cable.

Apparently, both devices would need to use the same clk + data lines,
so how does each device know when pc-commands are directed
at them, and not the other device ?

thanks very much,

Rob.