I was watching a video demonstrating a technique of putting a nail through two pieces of wood, bending the nail back on itself against a metal plate and hammering it in, so that the end result was sort of like a staple. What is that technique called?
It is called clinching. The technique is pretty simple, yet hold fairly well to prevent separation from the two boards.
Here is a good example of how to do it:
The example calls for a small bar to be used to bend a short length of the nail. This can also be accomplished by just using a hammer to angle the nail. Any deformation of the wood will be hidden by the nail itself when it is driven back into the surface.
Hammer a nail most of the way through both boards. You want enough of the pointed end of the nail so that you can fold it over. Using a small piece of metal to help shape the nail, hit the pointed end of the nail causing it to bend in the direction you want. You don't need to be absolute on the angle, approximately 90 degrees will do.
Remove the piece of metal from the workpiece. Drive the nail completely through the two pieces of wood. You may have to set the workpiece on something to give you clearance for the remainder of the nail. Strike the nail between the folded over section and the workpiece, closer to the folded over section. This will cause the nail to bend as in step 2. You should drive the nail either in the direction of the grain or across it. Different people have different attitudes about why you should drive in either direction. For me, it boils down to do I want the nail raised above the surface of the wood or not. If I do, then I drive the nail across the grain.
Drive the bent end of the nail into the workpiece.
Instead of using a hammer to bend the nail, you can use pliers or a vice. With thicker diameter nails, using an alternative method becomes problematic. One video I have seen recommended using a metal plate to turn the nails as you drive them.
Clinching or clenching. I've always used the former word.