What do you mean by save diamonds for extended energy?

The extended energy bar costs 799 diamonds and can be found by tapping the plus sign next to your energy count. It will remain a permanent feature of your game.You get 450 energy to start, energy that refills to 150 every 5 hours (instead of 110 in 3hr 20 min), and 150 energy (instead of 110) for every character puzzle you complete and every star booster you activate. Highly recommended for those who want to play more during Detectives Needed Key Challenges or Team Competitions.