Imagine you're trying to send someone a message on their iPhone, but they don't currently have an internet connection. For example, they may have run out of mobile data or temporarily have no mobile data coverage. In that case, you may prefer to send a regular SMS instead. Assuming, of course, that calling isn't an option.
If the recipient normally uses iMessage, or perhaps has other devices signed in to the same Apple account, such as an iPad at home that's connected to Wi-Fi, your message will usually be sent as an iMessage.
But if you're trying to reach someone who currently has no internet connection on their phone, it doesn't help much if your message ends up on an iPad at home. Nor is it particularly useful if the message only arrives later when they reconnect to Wi-Fi or mobile data. Sometimes you simply want to send an SMS that goes directly to their phone.
How do you do that?
Write a message to the person you want to reach. You'll notice that the text field still says 'iMessage' and that the send button is blue. That's perfectly fine. Type your message and send it.
Now comes the trick. Your message will appear as a blue message bubble in the conversation. Press and hold that message. A menu will appear. From that menu, choose 'Send as Text Message'.
You will now see the message twice in the conversation: first as the blue iMessage version and then again as a green SMS message.
But, if you close the Messages app and open it again, the iPhone will have removed the original iMessage version from view, leaving only the green SMS message.
Good to know
There is one important detail. If the recipient has already received and read your iMessage, you can no longer resend it as an SMS. If you press and hold the message, that option simply won't appear in the menu.
As the sender, you can also temporarily disable iMessage in your iPhone's settings. All messages will then be sent as regular SMS messages, indicated by the green send button and green message bubbles. Just remember to turn iMessage back on afterwards, otherwise your messages will continue to be sent as SMS from that point on.