How can I hook into entityforms being submitted?
Entityforms are - as the name suggests - stored as entities. So you can hook into hook_entity_insert(), or hook_ENTITY_TYPE_insert(), having the inserted data as parameter (the entity will be an object):

/**
 * Implements hook_entity_insert().
 */
function MYMODULE_entity_insert($entity, $type) {
  if ($type == 'entityform') {
    // Do your stuff here.
  }
}


or

/**
 * Implements hook_ENTITY_TYPE_insert().
 */
function MYMODULE_entityform_insert($entityform) {
  // Do your stuff here.
}


You could also utilize rules to define custom actions on submission, as the entityform submission triggers the entityform_submission rule.