MP is the slope—the rate of change—of the TP curve. When TP is rising at an increasing rate, MP is positive and rising. When TP is rising at a diminishing rate, MP is positive but falling.
MP first rises because the fixed capital gets used more productively as added workers are employed. Each added worker contributes more to output than the previous worker because the firm is better able to use its fixed plant and equipment. As still more labor is added, the law of diminishing returns takes hold. Labor becomes so abundant relative to the fixed capital that congestion occurs and marginal product falls. At the extreme, the addition of labor so overcrowds the plant that the marginal product of still more labor is negative—total output falls.
Diminishing returns requires increasing amounts of the variable input to produce each additional unit of output. At constant input prices, using more inputs will increase the cost of each additional unit produced.