Putting a baby down to sleep is a little like setting a mousetrap.
You must gingerly "set" both... for the baby, it's a fresh diaper, swaddle, a little soothing and a pacifier. It's analogous to putting the crossbar of the trap into the little slot, releasing the tension and hoping it holds.
Then, you have to quietly, easily, set the baby down. Much like a moustrap, if you make the wrong move...
If you're a parent and have ever set a mousetrap, you will get this analogy. Sometimes, in either case, it's best for the faint of heart to leave the room and allow someone more brave to handle things. Which is why, at 4:45 am, I'm in the kitchen writing this.