Every time you collect Goods, Coins, Energy, XP or Collection items by hovering over them within 6 seconds of them appearing, the Bonus Bar appears in the top right hand corner and starts to fill up. Once the bonus bar fills, it will move to the next stage and appear empty again. The higher the stage is, the more extra Coins you will get according to your level.
When the bar is being filled, no matter how many actions you take, they only count for one (until it reaches the next threshold). That's why collecting items in a slow pace is better than in a very fast pace. The best thing is to wait for the bar to stop growing to take the next action, but also taking care for the reward not to be collected automatically.
If you do not collect goods, coins, XP or collection items the bonus bar has a red glow that flashes around it for 6 seconds. This is warning you that if you do not collect anything else soon, your bonus will expire and you will collect coins based upon your current bonus bar level. Anything you do that requires energy will drop something you can collect that will increase the stage of your bonus bar, so the more energy you end up using at once the more things you can collect and the greater the bonus coins you will recieve from the bonus bar will be.
Threshold - Number of reward/item pick ups required to get the bonus and enter the next stage. 32 total required for a full bonus.