Ember.Handlebars.helper and Ember.Handlebars.registerBoundHelper extend registerHelper handlebars method with EmberJS specific features as data binding or view creation.
registerBoundHelper creates a bound helper.
Bound helpers behave similarly to regular handlebars helpers, with the added ability to re-render when the underlying data changes.
Ember makes available Handlebars instance with Ember.Handlebars ( i think, it is not recommended you access directly to the Handlebars instance), and it allow you to write specific handlebars method with the registerHelper method whose function parameter accepts the format:
Ember.Handlebars.helper('myHelperName', function(property, options) {
});
If you define your helpers using the registerHelper method, your helper will be in charge of observing data changes and re-render its data if it would be necessary.