Put your iPad, preferably in a water-proof bag, at one end of the pool. Set your interval time and the number of laps for your training. Hit "Start" and begin your relaxation watching the preparation timer.
Once the preparation timer reaches zero, start to dive your first lap.
While recovering for you next dive, watch the interval timer and start your next lap when it reaches zero.
Repeat until you have reached the number of laps.

App can as well be used for static training with fix-intervals for group training. Try a series of static breath holds of 5 times 3 Minutes. The more advanced divers will extend their breath hold time to 2:30 and shorten their recovery to 30 seconds, while novice divers can do a 1:30 breath hold and 1:30 recovery.

To train CO2-tolerance, set a decrement to reduce the interval after every lap.