You are right, an RBC must be extremely flexible in order to pass through capillary beds so the spleen screens RBCs to ensure that they are flexible enough to not block the body's microvasculature. The spleen does this by "mimicking" the capillary environment using the cords and forcing the RBCs to squeeze through them in order to get back into circulation. Any RBCs not flexible enough gets caught and digested by the macrophages. This is also why sickle cell patients have autosplenectomies and fibrotic spleens - the abnormal RBCs in sickle cell patients are too numerous for the spleen to handle and they block the flow causing thrombosis and infarction/fibrosis of the spleen. In any case, when the RBCs have these bodies (Howell-Jolly, Heinz, etc.), the majority of the RBC can squeeze through the cord but the bodies themselves are not flexible so they get snagged. In order for the RBC to keep going, the macrophages bite off the part that gets caught, freeing the RBCs and generating bite cells. Same principle applies for spherocytes which is when the splenic macrophages have bitten off so much of the RBC membrane that it can only take a spherical shape. The next time the spherocyte passes through the spleen, it won't be able to change shape at all and will be caught and digested leading to hemolysis in hereditary spherocytosis (which is why a splenectomy is used as a treatment).
