Rerender Example in Visualforce

This works.

<apex:page standardController="Position__c" title="Sample Position Layout Page" showHeader="true" sidebar="true" >
    <apex:sectionHeader title="{!$ObjectType.Position__c.label} Edit" subtitle="New Position"/>
    <chatter:feedWithFollowers entityId="{!Position__c.id}"/>
    <apex:form id="theForm"> 

<!--this is the level I am rerendering at...including all sections and children-->?
        <apex:pageBlock title="Position Detail" mode="edit" id="thePageBlock">
            <apex:pageBlockButtons > 
                <apex:commandButton value="Save" action="{!save}"/>
                <apex:commandButton value="Cancel" action="{!cancel}" immediate="true"/>
            </apex:pageBlockButtons>
            
            <apex:pageBlockSection id="detail" title="Information" columns="2">
            
                <!-- this field is also required...both at the field level, and in the markup-->
                <apex:inputField required="true" value="{!Position__c.name}"/>   
                
                <apex:inputField value="{!Position__c.Priority__c}"/>
                <apex:inputField value="{!Position__c.Location__c}"/>
                <apex:inputField value="{!Position__c.Status__c}"/>
                <apex:inputField value="{!Position__c.Type__c}"/>
                <apex:inputField value="{!Position__c.Sub_Status__c}"/>
                
                <apex:pageBlockSectionItem>
                    <apex:outputLabel value="{!$objecttype.Position__c.fields.Department__c.label}"></apex:outputLabel>
                    
  <!--this isolates the validated data-->                  
                    <apex:actionRegion > 
                        <apex:inputField value="{!Position__c.Department__c}">
                            <apex:actionSupport event="onchange" rerender="thePageBlock"/>
                        </apex:inputField> 
                    </apex:actionRegion>
                    
                </apex:pageBlockSectionItem>
                
                <apex:inputField value="{!Position__c.Start_Date__c}"/>

<!--...more fields...-->

            </apex:pageBlockSection>
            
  <!-- this is the conditional section -->          
            <apex:pageBlockSection id="theSection" title="Technical Skills" columns="2" rendered="{!Position__c.Department__c == 'IT' || Position__c.Department__c == 'Engineering'}">
                <apex:inputField value="{!Position__c.Operating_Systems__c}"/>
                <apex:inputField value="{!Position__c.Programming_Languages__c}"/>
            </apex:pageBlockSection>
            
            <apex:pageBlockSection title="Description" columns="1">
            
  <!--this field is "Universally required" at the field level-->
                <apex:inputField value="{!Position__c.Job_Description__c}" style="width:300px" />

                <apex:inputField value="{!Position__c.Responsibilities__c}" style="width:300px" />
                <apex:inputField value="{!Position__c.Skills_Required__c}" style="width:300px" />
                <apex:inputField value="{!Position__c.Education__c}" style="width:300px" />
            </apex:pageBlockSection>
        </apex:pageBlock>
    </apex:form>
</apex:page>