I think it was recently added, like in the last year or so. Glad you like it - I could never get used to it, but see how efficient it could be.
Just put your digit on the first letter of the word you want to type, then swipe it along to the next letters in sequence, without lifting it up. It leaves a momentary trail showing the path. Lift up when you’re done the word, and it should pop up on your screen. Then put your digit on the first letter of the next word, and repeat.
On the iPad it only works on the mini keyboard. You need to bring up the regular keyboard and then pinch together with two fingers to bring up a keyboard similar to the iPhone. Swipe them works as expected.
Thank you~will try to teach this old dog some new tricks. 
Good luck! I just learnt to do it yesterday so there‘a hope for you!
Great instructions! Thank you. I’ll be trying it tomorrow after I’ve downed my first mug of coffee 
Thanks for the tip about the mini keyboard; I had no idea the iPad had that feature. I have an issue iPad Air 3 for work, but don’t type much on it.
But how in the heck can you hold the iPad in one hand and use the same hand’s thumb to type?
Portrait or landscape the keypad is placed in the center of the bottom margin and isn’t movable. I’ve got decently large hands but no matter how I hold the iPad, my thumb can’t reach the whole mini-keyboard and the iPad is trying to fall out of my inadequate 4-finger grip. ANd I even have non-slip tape all over the back to make it less precarious to hold onto.
Because what I wrote was confusing 
My earlier post I was typing on my iPad while discussing my iPhone. Obviously the techniques are completely different and I misremembered my iPhone technique.
On my iPad I normally type with one finger if I am holding the iPad, or multi-finger if is is sitting on a flat surface. I just discovered the mini keyboard which allows swipe. There is also a split-keyboard mode which allows more comfortable thumb typing with both thumbs at once.
Aaah, thank you. I’d noted the inconsistency but was hoping it didn’t mean what it seemed to mean.
One of my hoped-for use cases is to carry the iPad and type on it one-handed while also walking. I haven’t made any progress on that. Here I was hoping that you’d figured out the magic Spock grip or whatever that made it possible and could share your discovery.
Huh, maybe i could try using an iPad again. Someone gave me one years ago, and it drove me nuts. Mostly, it was just that it’s very different from Android, so most everything i wanted to do was unintuitive. But partly i hated, hated, despised typing on the thing. Not the size of a real keyboard, too large to use one handed… just… wrong.
Tangentially related, I highly recommend Pop Sockets. They make holding onto your phone much easier, and save you from cramped and tired hands.
Years of snapping spring clips and plastic spread rivets on factory assembly lines damaged my thumbs. I do my screen typing with my right index finger or a stylus. If I have access to a real keyboard, I revert to old-school touch typing. I’m no speed champ, but I can touch-type much faster than picking out letters with one finger.
I use my index finger. Although i saw the title of this thread and thought I’d give the thumb business a try. Awkward, but it can be done. (don’t do it over the side of a boat though) 
It’s easy enough to download a different keyboard. I downloaded Gboard (Google Keyboard) almost immediately on my iPad.
When I got it, Apple didn’t allow alternative keyboards. They really are a security risk – your keyboard sees everything you do. Even after they allowed it, non-Apple keyboards were crippled, and people I knew who used them were frustrated. I can believe they have solved those problems, but I stopped using the iPad long ago.
Yes, I type with my thumbs on my smartphone. 
I have an android phone (Samsung Galaxy s9+) and a 6th generation iPad.
On my Galaxy, I type with both thumbs and the phone in portrait format, and I’m fast at it and very accurate. I have small hands, and typing in landscape format is doable, but not as comfortable - I have to rest the phone on the tips of my fingers and really stretch with my thumbs.
Early on, I swipe-typed for awhile, but I found I made more mistakes that way, and I can’t stand to leave a mistake uncorrected which slows down my typing speed. It also didn’t feel as comfortable to me holding the phone in one hand and swiping (or tapping) with the other. So, thumbs it is!
Small hands are more of an issue for me with typing on the iPad - even portrait mode is too wide for me to hold it and thumb-type. I’ve only had it for about 6 months and have fiddled around with different ways to type on it, none of which really felt intuitive after years of using an android phone.
What I eventually fell into is using my forefingers to type with the iPad either on my lap (propped on a small pillow) or the arm of my chair. Since I’m not actually holding it while typing, portrait vs landscape format doesn’t matter much, though I prefer portrait.
My accuracy isn’t as good as with my phone, and due to the very different keyboard layout I have to look at my hands a lot more often while typing, which I almost never need to do on my phone. That doesn’t matter much, tho. I mostly use the iPad for reading and playing word games, so I don’t spend nearly as much time typing on it as I do my phone.
I never even thought about downloading a different keyboard for the iPad. I didn’t know it was possible for an Apple product. 
Gonna try that! 