That leaf spring appears to be part that helps hold the thread in place.
It's sort of "self threading" as you pull the thread at the top, and it slips under the leaf spring, then drops down and is held in place. Since it sits on top of the metal that is part of hte needle bar, I assume that it acts as some sort of tension device, but maybe not. As the needle bar goes up, it's going to raise the thread into the area where the leaf spring is in contact with that bar, and I assume that adds tension of some sort - have not sewed on the machine so I've never watched what it actually does.
Here are the pictures - you can do the math LOL!!
