I use one of these quite a lot. My great grandfather called it a hand scythe. I'm not certain what the modern word for it is - most of the tools he used now have new words.
I sharpen it like a scythe, maybe even a touch sharper. My handle is bent a bit different, but the motion is like a scythe.
Get the clump of tall grass, hold the blade parallel to the ground, swing the hand scythe back and to the right, take a gentle swipe at the tall grass, about an inch or two above the ground. If the 'bite' of grass is about the width of the blade, then it give just the right amount of resistance. Too little grass and it swings too freely, too much, and it puts strain on the wrist. Swing fast, but controlled (it's really sharp) and sharpen every time it starts to put effort on the wrist.
Unlike a sickle or most other hand tools for cutting
straw, you
don't grab hold of the grass/straw when you swing the hand scythe. Righthand goes back, lefthand only goes in if it needs to help the grass fall, otherwise, it stays out of the way and it out of there long before right hand brings sharp thing back towards the grass.
This is my favourite tool for steep slopes that are difficult with a scythe or whipersnipper. Hint, work the way up hill. I'll use this for five to 10 min a day to gather fodder for the livestock, but I am considering trying it for harvesting grain this year.
That was how I was taught to use it. There might be other ways from different parts of the world.
Sorry, stuck in bed sick today, so no video, but here's a (really bad) drawing.
Edit to add: google gave me
this link when I searched 'sickle scythe'.