Sound complicated? It is, and if you mess it up you can end up spilling milk (though it shouldn't be much milk, as it's only what's left in the shield and tubing). I found myself opening the app to get the instructions to make sure I got it right, and once I did forget and spilled some milk in my kitchen. (No, I didn't cry over it, but I did consider it.)
The Willow works this way because by default, it comes with O-shaped milk bags that you place inside the pump before your session. It then seals the bag shut when your session is over, with every drop of milk you pumped. And while that's handy, especially if you want to pop your milk straight into the freezer, the O-shaped bags weren't the easiest to get the milk out of, and there was always a little milk stuck in the bag's various crevices.
There's also a reusable container you can use instead, which worked well (and still required you to flip to finish). I was worried it would be hard to clean since it's also an O-shaped item, but the middle pops out to make it much easier to clean after you pump. And it's dishwasher safe, too. But you will need to have somewhere to pour the milk into after using the reusable container (such as a baby bottle or freezer bag) since it's not designed to store milk outside of the pump.
Extra, Extra
Willow sells more accessories than other wearable pump makers. I tried Willow's pumping bras, which have a clip to lower the bra cup, place the pump, and then re-attach the cup on a lower clip to secure the pump in place. It's similar to a nursing bra, and while they weren't more secure than my other nursing bras, I did find them comfortable to wear in general.
The Willow 3.0 is on the heavy side, and it's large, too. With the silky Willow bras, I found the pump tended to pop upward out of the bra rather than stay in place. I preferred pairing it with a nursing sports bra that had larger, more secure bra cups to hold it in place and keep it from slipping around.