I do acknowledge that this statement is true, but I don't think it should be. Most 'buoyancy' issues are not related to buoyancy at all; they come from poor technique in the water. I see every day divers of all skill levels and I refuse to carry extra weight, if a diver is having problems staying down help them to stop kicking themselves to the surface, or holding too much air in their lungs, or dump air from their BC, etc etc etc. Why do I weigh 250 lbs and dive with 6 lbs of weight while the muscle bound guy weighing 175 needs 18 lbs (in warm Hawaiian waters). Just something to think about as you dump 'extra' weight into your diver's BC
Ouch - I just cannot agree with this - buoyancy is a matter of physics and you cannot change it! - yes you can have other habits or problems that cause you to not sink, such as finning upwards etc.
BUT if you have not taken enough weight with you to counter your air use, then no amount of dumping air, breathing out or anything else will keep you down when the air in your tank depletes.
You
ARE going up, going head down and finning might help slow your ascent, and yes, you may be able to hold your depth by keeping swimming down, but that is no way to do a safety stop, the extra exertion at the end of a dive could increase your risk of DCI. I'm afraid saying I can do it, so other divers can do it as well and I won't carry extra weights, ignores the fact that many vacation divers do not have your levels of buoyancy control, may dive without doing a proper weight check and therefore not take enough lead, therefore it is sensible as a working DM to have bit extra.
As an aside when I do a drysuit dive with a newer diver I almost always take extra lead so I can help swim their legs down and prevent a runaway ascent when they have managed to to add too much air to their drysuit and inflate their legs, go head down and not able to dump air. Yep a drysuit diver is taught to deal with this, but when a newer diver doesn't correct it straight away it is reassuring that I can make myself negative enough to be able to easily help them get under control.
For the OP I think overcoming your problem is a question of practice, this is the first time in any diving course that you have been asked to deliberately overweight yourself, up to now the emphasis has been on getting yourself perfectly weighted. Coping with the extra weight is just a matter of practise and getting used to it. I prefer to dive with my correct weight, but if I have to add some then I don't find it too hard to manage my buoyancy. But then I spent a lot of time on shore dives taking extra weights in order to set out markers for navigation courses and so on, so have had plenty of practise of carrying extra weights, then dropping them, adjusting my buoyancy, doing it again, and then at the end of the exercise picking the all up and swimming then in again.
Try something like that - do a shore dive and carry three or four extra weights. Put one down, adjust buoyancy, and swim on, repeat and adjust buoyancy etc. etc. Then turn around, and collect them all up again, adjusting buoyancy each time. Do that a few times and it will start to become second nature.
Good luck with DM training - Phil.