You can set whether a section is collapsible or not in its settings on the Edit page and you can specify whether the section will be expanded or collapsed by default.

After that, if you want to expand or collapse sections with JavaScript, you'll need to assign the section a class via the Advanced tab of its settings, or locate its ID in the Preview Pane on the CSS and JavaScript page.

Here's the code that will trigger a section to toggle whether it is expanded or collapsed, assuming the section's ID is q2.

$('#q2 .collapsible').trigger('click');

Here's what it looks like in a working example. In this case, the code checks to see if a field's value matches "Eric" and, if it does, the code toggles the section.

$(document).ready(function () {
  $('#q3 input').blur(function () {
  if ($('#q3 input').val() == 'Eric')
  {
    $('#q2 .collapsible').trigger('click');
  }
  });
});

Note that you can also use the .show() and .hide() methods in jQuery to hide and show aspects of a form.