Forms :: Executing Query When Display Box Value Changes

In my form i have 2 datablocks, the first contains only a display item which is populated from a lov when a user presses a button. The second datablock shows a list of items which should be queried dependant on the value of the above LOV. What i thought i could do is on a post-change trigger for the display item is:

go_block('block_name');
do_key('execute_query');

However, i am told i cannot do these in a post-change trigger, how to acheive what I am looking. Would i perhaps have to create my own trigger somehow or is their a simplier way.