Casinos use what are known as shills on poker tables. When a table is reduced to say three or four players, they join the game to play and just check and maybe call bets to keep the hands going. They never raise or try to win, when you recognize this, leave the table.
I have played hundreds of times in poker games in Vegas and elsewhere. Most of my playing history in smaller poker rooms ended around 20 years ago. After that I usually played in bigger poker rooms. Shills and prop players were very commonly in evidence in the poker games in the smaller poker rooms 20 and 30 and 40 years ago [ generally less than 5 poker tables ] to keep poker games going when they became short handed , often late at night or when the room first opened in the morning .
A proposition [ prop ] player plays with his own money and is told to enter a short handed game at the poker room managers direction. He is an employee of the casino and is paid by the casino . Any wins or losses were footed by the prop player . This would often include poker dealers and floormen who were not dealing at that time.
A shill player was also a poker room employee but they played with house money and the casino takes any losses or wins. These players usually play very carefully.
Shills and prop players as far as I was concerned were pretty much the same thing and once they got in the games it meant that it was time to leave the game since the action had dried up.
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Whether the rules have changed I have no idea. But these players are probably near extinct today since almost all poker in casinos is now played in bigger rooms, often with players on waiting lists, and most all of the small poker rooms in lower- end casinos have been closed for years. So the need for shills and props is very little.