This is a CMOS transmission gate, which acts as a switch. When the switch input is high, the 40 Hz signal can flow through the transmission gate. When the switch input is low, it can't; the transmission gate acts as an open circuit.
Normally a discrete MOSFET has its body terminal connected to the source terminal, but in this case that doesn't work. The source terminal of the n-MOSFET may be higher voltage than the drain (and the p-MOSFET's drain may be higher than the source), which would cause the body diode to conduct. So instead, the MOSFETs have to have a separate body terminal tied either to ground (for the n-MOSFET) or the supply voltage (for the p-MOSFET).
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