"What's that thing around your neck?" It's a common question that I receive from beginner freediving students as we sit together in the boat, speeding over the sea toward the dive site of the day. They've just been issued with rubber belts and lead blocks, fastened carefully with the weights in the front so as not to scratch the paint on the boat. It doesn't occur to most of them that this big collar I'm sporting could be for anything other than some kind of special freediving fashion that they haven't discovered yet. "It's weight," I respond, with a smile. "In deeper freediving, it can be nice to move some or all of the weight to your neck so that you have a better position in freefall...and when I'm teaching, it helps me maintain a better position to follow you down the line!" This has more or less become my default answer to the question that I receive every three days or so. But the curious students sometimes persist. "Isn't it uncomfortable?" "But you still have a belt on..." Alright, students, this one's for you! Let me elaborate!
I stand by my original answer. When you perform a deep dive with a long freefall, putting weight on the neck rather than the hips can help you to maintain a better body position. You've moved the densest portion of the diver from the hips (a point halfway up the body) to the neck (a much smaller, more narrow point at the bottom of the body, when the diver is inverted). It's a lot about balance. Imagine a few pieces of lead dispersed on a belt, worn on the hips. Unless those blocks are perfectly dispersed, any small offset will pull the body this or that way, and the diver will have to compensate with micro-adjustments. While I have certainly witnessed some divers performing deep dives using a weight belt, and having seemingly mastered these micro-adjustments, my experience has been that it's much easier to create a balance requiring less bodily adjustment with the weight on the neck. The neck is small, and an oval, properly-fitting neck weight will sit comfortably and perfectly balanced.
On deep dives with long freefall, a neck weight can be more streamlined than a belt, especially if the belt has an extra bit of length that likes to flap around during the dive. While there are solutions for preventing that awful belt-flapping problem, having blocks of lead jut out from the hips is still probably less streamlined than a smooth oval shape around the neck. While it may not be a huge deal for many dives, I've noticed that sometimes, even the tiniest changes in streamlining can result in precious seconds added or lost on deep dives.
Good question. On land, it can feel strange to put a neck weight on. In the water, upside-down, my experience has been that you get used to it pretty quickly, especially if you have a well-fitting neck weight. I've found that the most I can comfortably tolerate around my neck is about 1.36kg / 3lb, which, thankfully, is the amount of weight that I use for my deep diving in 1.5mm wetsuits. However, when I'm teaching, I'm wearing a thicker wetsuit. This brings me to...
When I'm wearing my full 3mm "working" suit, I need about 1.8kg / 4 lb to be neutral at approximately 12m (this is where I like to set my neutral buoyancy), and when the suit was brand new, I actually needed 2.27 kg / 5 lb! Now, I know this is nothing compared to friends I've spoken to who dive in 5mm or even 7mm+ suits in colder waters (much respect), but for me, it feels like a lot.
I have two options to disperse the weight: put it all on a belt, or put some on a belt and some on my neck. I go with the latter option, because putting all of it on a belt seems to make it much more likely that the belt with move and slip while I'm following students down the line, inverted. Moving a portion (in my case, most) of the weight to the neck allows me to wear just one or two tiny weights on the belt, and my belt stays happily put.
In this situation, I won't put all the weight on my neck, as anything more than about 1.5 kg starts to feel like a lot, and I've got to wear it for some time on land plus a boat ride before I'll be relieved in the water. I've met others who are used to wearing a bit more on the neck. It's all about what you're used to.
And finally, let's not forget the obvious reason you might choose to wear a neck weight: they look cool. And we're suckers for looking cool (or rather, thinking that we look cool) in freediving. Come on, you know it; we have these smooth skin colored or perhaps camo wetsuits and crazy comic-book-superhero-looking masks. Add a thoughtfully-designed "collar" and you've got fashion, baby.
You don't have to be an instructor or a deep diver to try out a neck weight for the first time. You can try it for practical or aesthetic reasons, and anything that's going to help you be more comfortable in the water and during your dives is an obvious plus. If you're a diver who requires a lot of weight and you normally wear it all on a belt, you could consider moving some of it to your neck, which might be a more comfortable solution for you and help the belt to stay in place better. Experiment, and find the solution that works best for you!