Emails Control Panel:
When you are viewing an Order in WooCommerce the WooCommerce backend you will now have the Email Customizer panel
Allows you to choose which of your Email Templates you'd like to Send or Preview
Preview Email Button:
Once you've selected your email template above you can click this button to popup it's preview.
This will take you to the preview of the email you choose, so most importantly you can see how it will look to your customer
Click the ▼ to see the Header Info that will be sent with the email - Subject, From Email, From Name, To Email.
Click the to close the preview to return to the Order.
Send Email Button:
This initiates a send of the selected template to the Order Billing Email address.
You also have a chance to change the sending address before sending - e.g. you may wish to send yourself a test before sending to your customer