in the sub report you create a formula with a shared variable assigning the value you want to return
shared stringvar x;
x: = (your yes no value field here)
in the main report create another formula field and do the same thing but assign it as
shared stringvar x;
x:=x
this moves the value from teh sub report to the main report
you can now reference this formula field in a suppression statement
eg. @formula='Yes'
note that sub reports cannot be suppressed and still run.
there is a trick to this. Suppress all of the fields in the subreport, set the sub report to suppress when blank and then set the sectio it is on to suppress when blank. It still runs this way.