You can find a diode connected NMOS (M2) and a common source NMOS (M1) in your textbook.
The source of M2 is connected to Vout, so there is body effect in the M2 device. In this configuration, Vin will affect both Vout and Vth2, and the Body effect is related to the value of VSB, so it can be said that Vth2 and Vin are dependent.
This formula "δVth/δVin = η(δVout/δVin)" tells us that if the body effect is ignored, the gain of this configuration is δVout/δVin=sqrt(W1/L1)/sqrt(W2/L2); if the body effect is considered, the gain of this configuration is δVout/δVin=(sqrt(W1/L1)/sqrt(W2/L2)) * ((1/(1+η)), the gain will be decreased.