Postillion as the ‘cut card’?

A postillion is the 'cut card' that protects/hides, the face of the bottom card in the pack in card games such as poker.

It would appear that as a postillion sat adjacent to (or rode with) the driver, and as his job was to guard the driver and coach, then the card that sits next to the bottom card is next to (or accompanying) the bottom card (to protect it from being seen.)

If you have 3 aces and you can see that the bottom card is an ace, you know that nobody else has an ace, etc.

Ever heard of a Postillion sentence? This is one...That table has four legs...It's a sentence that would never be used.

This Postillion in poker does hide the bottom card and is often called a cut card but it's a postillion, wait for it...A card that will never be used in the game...i.e. never dealt out. Nothing to do with horses.