Breath-holds feel and are different from day to day. There are many influences, rest, time, food, water, preparation, relaxation, mental state. The large amount of factors make it challenging to learn and be consistent.
Hyperventilation is breathing more air then you naturally need.
Beginners often do this by increasing the respiration volume, deeper breaths.
To find out, you need to explore your CO2 range, testing it safely on land.
Knowing educated and top freedivers will avoid hyperventilation, we can do a dry land test to learn how it feels. We're going to contrast and compare
no breath-up preparation with
hyperventilation. To prepare for this test you want to do first 3 warm-up holds, so you'll get comfortable and reach your baseline performance. I suggest doing on land (dry) active exhale holds, because 't gets results faster.
Start with 3 active exhale breath-holds to 50%, 70 and 85% of your maximum. - active exhale is breathing out everything without reverse packing. 85% is a
feeling, where it feels like you could have done 15% more to meet your maximum. I use the percentages because time varies with the days, and trying to meet a specific time may cause training being too hard, and creating a negative association with long breath-holds.
So after the first 3 holds,( on land off cause! ) with enough rest in between, you're used again to higher CO2 levels and contractions.
Now let's start
with hyperventilation, and see and record what happens. Do say 2 minutes of deep breathing, but try to avoid tiring yourself and getting too dizzy. Then active exhale and hold your breath. Remember to do everything else as normal. Do another after some recovery time and again 2 minutes of hyperventilation. By now you know how this feels, during preparation, in the beginning, middle and then end of the breath-hold, also take note of the recovery time and feelings. Notice the effects on your physiology and mind.
After the 3 warm-up and the two hyperventilation holds, take 5-15 minutes rest, like a short sleep, and when you wake up notice your body's relaxation, do a no-breath-up hold: active exhale, hold. - it will be mentally challenging because you believe you're not ready. Just take it slow and relax, notice and record the experience, like with the hyperventilation holds, but avoiding analysing, just record the sensations.
After your recovery take notes, and compare hyperventilation holds with the no-breath-up hold.
Now the difference between both preparations and effects should be much more clear. When you do your next pool session (with buddy!) you'll have a much better sense of if and how much you've hyperventilated because you've got the experience of the two extremes.
Again only experiment with hyperventilation on soft land, and being rested, hydrated and in good health.
If you feel you're getting (too) light headed, lay down stable on the side (if you're not already laid down!) and hold your breath and wait for the balance in your blood to restore.
Because of our stressful lives, and bad breathing technique examples around us we mimic, many people have developed the habit of breathing with the chest only too much. Freediving most often has a very therapeutic effect of (re)learning the correct and most efficient diaphragm breathing in combination with feeling comfortable with elevated CO2 levels. Higher CO2 levels help in O2 transfer to the body's cells, improving oxygen availability in cells helps with many bodily functions, including healing and cognition.
Remember CO2 is good for us.*
*and for plants too, they grow faster.
ps, taking note of time during holds is distracting and difficult. What you can do is record a video, where you signal to the camera: breath-hold number and start, when urge to breath starts, when 1st contraction starts (if it's not visible), and possible other signs like bloodshift, heart rate, sensations of local heat, cold etc. This helps you to not think about time, but rather focus on relaxing and recording sensations, by relaying them to the camera with a small hand signal.
If you decide to put the video on Youtube, please add clear warnings and a link to Deeperblue.com so interested people learn how to explore and enjoy freediving safely. Thanks
/edit: expanding and clearing up.