<h:form >
    <p:commandButton class="btn" value="mostrar foo"
        action="#{developerController.foo}" update="foo"/>

    <h:panelGroup id="foo" rendered="#{developerController.bar}">
        foo
    </h:panelGroup>
</h:form>

import java.io.Serializable;
import javax.enterprise.context.SessionScoped;
import javax.inject.Named;

@Named(value = "developerController")
@SessionScoped
public class DeveloperController implements Serializable {

private boolean bar;


public DeveloperController() {

}

public void foo() {
    bar=!bar;
}

public boolean isBar() {
    return bar;
}

public void setBar(boolean bar) {
    this.bar = bar;
}
}