How to Minimize (& Maximize) Emails in Mail App on iPhone
You’re using mail on your iPhone writing a new message, but find you need to refer to something in your inbox or one of your folders. You scowl and close the message, save as a draft, lookup when you need, open the draft again, and finish writing. Apparently you don’t have to do that anymore in iOS 8. Minimize the new message, look up your stuff, and continue. Just hold and swipe the new message down and then back up. (If you did need to go to a draft, you can also press and hold on the drafts icon on the bottom right).
Source: How to Minimize (& Maximize) Emails in Mail App on iPhone