They lifted or removed the relevant piece and stepped aside
Despite the popular image of knights simply soiling their armor, that is mostly a myth. Plate was articulated, and the crotch and rear were left unplated so a rider could sit a horse, protected only by mail and skirts. A knight could lift his faulds or drop his hose and squat, often with a squire's help. Knights also did not wear armor all day, only in battle or tournament. Only in prolonged combat, with no chance to step away, might a man have no choice but to go where he stood.
