please have a look at http://community.qlik.com/message/149230
(There are also some other threads about this, but this gives quite a good overview).
2) You could use an action Selection - Clear other fields
attach is an example
when you click the button it selects all the values <=5 in field F1
then select the possible in F2 then clears the field F1
Example.qvw 156.5 K