Donny has explained most of this when shovels first came into existence, but I have still seen different people attempting to provide hints for others or myself about how to properly use any of the 3 different shovels; so I would simply like to help clarify.
1) Digging Shovel
Digs up various amounts of coins or flow at random.
2) Coin Shovel
Digs up various amounts of coins at random.
3) Flow Shovel
Digs up various amounts of flow at random.
Tips & Tricks
There is NO secret to digging up coins or flow.
There is no special location or section of ground that can guarantee more coins or flow. A common idea and/or urge is to continually move to different areas in hopes of finding the sweet spot, but digging for both coins and flow is simply a game of chance.
(Pro-tip: Dig only where you could dig IRL…j/k)
The best tip for digging is to dig wherever is most convenient (where you are at the moment) or to select several of your favorite locations and dig at each until you receive your treasure (Note: You can only dig up coins/flow at each location once per day).
There is also a maximum amount of coins and flow one can obtain per dig (e.g. Flow shovels can dig up between 3 and 120 flow).
The exact limit of coins and flow that can be dug up per day has not yet been determined, but after finding “treasure” at a few different areas, digging will cease to provide any rewards. However, if you are certain that you have not dug in 24+ hours, then you will eventually find coins or flow, it’s only a matter of how long you dig and how lucky you are that day.