When 3.3v Tx is transmitting a logic 0 (0 volts) the mosfet channel is "on" and the 5v Rx is driven low. When 3.3v Tx is switched high (3.3v) the mosfet channel is off and the 5v Rx is driven to 5v (logic1) through the 10k pull up resistor. Note 10 ohm resistor is for simulation purposes only to be able to use physical switch. Not needed in actual circuit.
