As a college athlete (Football and Wrestling) and only a newbie Freediver, my area of knowledge lies much more in weight training than in apnea and freediving. So while I cannot necessarily relate weightlifting to freediving (as I can for football and wrestling) I can share some insight into lifting. Granted, most of ya'll are lifting for different purposes than I am, but many of the basics hold true for both explosive/mass building weight training (what I do) and general fitness (what many of ya'll engage in)
Although everybody has their own theories on what works for them there are some tried and true concepts that should be adhered to. I cringe when I read about people holding their breath while lifting, probably just as much as the moderators of DeeperBlue do when people write about freediving alone. Whether holding one's breath while lifting is beneficial for freediving is not the true issue here. Ya see, there are other ways to improve on that. It should not be done in the weight room. Proper breathing while performing any lift, no matter the sets/reps/percentage is just as important as proper form throughout the lift. Honestly, I have read about this as much as ya'll have read about freediving. Holding your breath is one of the most deafting things you can do to yourself in a weight room.
Basically, the point here is that while it may or may not improve your apnea, it's pretty much the worst way to go about doing that. It's kinda like steroids - yeah they can work, but overall it's pretty bad. I'm sure there are other ways to improve apnea times. Plus proper breathing while lifting it will make your workouts much better - I promise. Just like with freediving - it is important to breath up properly.
Tyler