The rock will break if you’re currently powered up from fruit – open your inventory and you’ll see a little counter. If you want to “burn off” unwanted fruit charges, use your shovel to lift up (and replant) whole trees.
So long as you’re not all pepped up, dig the lockdown holes as above, position yourself, and whack away with A as fast as possible. You can get 8 minerals out if you are quick. Once a set amount of time passes from your first tap or two, the rock will “expire,” essentially, hence the need to anchor yourself for maximum button-mash-age!
Once a rock is “tapped out” for the day, you’ll need to wait till the reset time (5AM, IIRC) to try again.