When Red was teaching Thomas to ride broncs, what trick did he play on Thomas?
Red's plans for Thomas include using Thomas to make money for Red. Red tells him when to win and when to lose in order for Red to increase the gambling odds in his favor. All Thomas represesents to Red is a way to make money. He uses him, not caring what effect this has on Thomas. Red is greedy and selfish and manipulates Thomas to ride for the sole purpose of making money for Red. He is able to exploit Thomas because he knows Thomas needs him at first to teach him and provide him with horses.
Red wanted Tom to learn never to trust someone else with his gear, so REd cut the latigo and the cinch of Tom's saddle before Tom mounted. As Tom started to ride, The cinchs gave way and the saddle turned upside down.
In order to teach Tom the importance of checking his own gear and ensuring that no one tries to sabotage him during a ride, Red cuts the latigoes and the cinches slightly so that when Tom is riding, his saddle falls apart on him causing him to fall off the horse. Red then tells Tom that he gave him more than one chance to check his gear. He says, "Don't trust no one with your gear, not even me."