1. Repeatedly inflating and deflating BCD
One of the things you’ll learn from the theory is how to check for neutral buoyancy: on the surface, you should be eye level with the water with no air in your BCD.
It is important to check for neutral buoyancy at the beginning of your dive so that you avoid repeatedly inflating and deflating your BCD underwater to stay at the correct depth. With practice, you will be able to ascend and descend a few feet using only the air in your lungs.
It’s important to note that just because you used 4kg on your last dive, it doesn’t mean it will be the same for the next. Weight gain or loss, wetsuit thickness and water salinity all play a part so make sure you adjust accordingly.