What does the detail of Joseph's brothers sitting down after throwing him into the pit point to?
It foreshadows the scene at Christ's crucifixion, when after He was put on the cross, they sat down and watched Him there. Joseph's brothers cast him into the pit, effectively killing him in intent, then sat down, providing a perfect picture of what would happen thousands of years later.
Les explains that after Joseph's brothers threw him into the empty pit, for all practical purposes they were killing him, and then they sat down. He connects this detail directly to the crucifixion: when Christ was put on the cross, Scripture says they sat down and watched Him there. Les calls it a beautiful, perfect picture of what would transpire thousands of years later. The added note that they sat down to eat their bread further underscores the scene.
And it came to pass, when Joseph was come unto his brethren, that they stript Joseph out of his coat, his coat of many colours that was on him;
And they took him, and cast him into a pit: and the pit was empty, there was no water in it.
And they sat down to eat bread: and they lifted up their eyes and looked, and, behold, a company of Ishmeelites came from Gilead with their camels bearing spicery and balm and myrrh, going to carry it down to Egypt.