What's the role of the body-source resistor？

[coolsummer]

Hi, I saw a common use of the body-source resistor in RF power amplifier designs, what's the role of it? Somebody says it can block rf signal from entering the body of the cascode transistor, but if the voltage of node X is swing, the voltage of node Y will also swing since the resistor R is large and there is barely no current flowing through it, so how does it prevent the RF signal and what's the role of it?

 

[dick_freebird]

R and the Cbs (if junction isolated) forms a low
pass filter, the body will assume the average
source voltage rather than following it, which
can keep the body from pestering the drain
with opposite-phase RF signal (or vice versa).

 

[coolsummer]

R and the Cbs (if junction isolated) forms a low
pass filter, the body will assume the average
source voltage rather than following it, which
can keep the body from pestering the drain
with opposite-phase RF signal (or vice versa).

could you tell me how do R and Cbs form the low pass filter? I draw a picture and i1 is the current produce by M1. It doesn't seem to be the low pass filter example in the right half of the picture since node Y is floating?