hope this will help to understand what palette snooping is used for this primarily relates to multimedia devices ie... by enabling a ISA MPEG card can be synchronised with PCI/VGA.
When a PCI device intercepts a cycle, this cycle is not forwarded to the ISA bus. In the case of VGA palette write cycles, this may be a problem if the VGA controller is a PCI device and an ISA device needs these palette write cycles to adapt its internal palette with the VGA requested one. To allow this, the PCI VGA devices have to implement this feature, asking them to snoop the palette write cycles, not to intercept them.
Because the internal VGA is connected to the host bus, dedicated cycles to it are not forwarded to the PCI bus. If a PCI or an ISA device needs VGA palette write cycles, the internal VGA has to snoop them, not to intercept them. This feature is enabled by setting the bit 2 of the register 29h.
Note: the internal VGA palette snoop feature forwards related cycles to PCI. You have to enable the PCI VGA palette snooping to also forward them to ISA.