To learn what it means to relax your hip flexors, squat down onto a chair or short stool or something, then when you're on it, lift your feet off the ground for a split second, then stand back up. This places all your weight on the 'box' and gets rid of the energy you store up while descending.
This is a large part of why a box squat should be significantly lower than a free squat.
To understand the mechanical disadvantage I mentioned in my prior post, you have to understand the most mechanical advantage is achieved with both knees and hips in line with the mid-foot. In a Westside style box squat, your knees are behind mid-line and your hips are as far back as possible without falling over or sacrificing good form.
That's another reason a box squat should be harder.
It's a great movement for explosive hip drive out of the hole and developing the posterior chain. This doesn't mean it's the end all be all movement. Even done properly, this movement isn't optimal to include in everyone's program. On the other hand, it might be something you always want to include. Check out some of Dave Tate's literature and videos on the box squat. There's a lot out there.
Also, make sure you push your knees out hard. Knees in line with your pinky toe. Most people won't do this without a lot of cueing and practice at first. This will keep you more upright so you don't do something looking like a wide stance goodmorning to a box.