Hmm... I don't see why you would want to bring the racquet down first and then up instead of just up.
When you initially bring the racquet down, it actually goes up in the takeback regardless. This is because you're causing the front shoulder to dip and the back shoulder to rise. You don't need to consciously lift the racquet into a loop -- it happens naturally as you take the racquet back. Main thing is whether the elbow starts above the racquet at beginning of takeback. That's one tell that somebody's using a pendulum takeback.
The main reason for doing this is to prevent the need to swing across the body. When you execute a traditional loop, it starts with the arm moving away from the side of your body. Your swing is loaded to do that. If you then try to execute a forward swing in a C-loop (without the arm coming across), you'll notice that the contact point is brought in and that the swing doesn't drive through. With a pendulum takeback, it lets you swing all the way through. Downside with a pendulum takeback is that you have less power on high bouncing shots and less spin variation.