The sweet spot: that ...

The sweet spot: that productive, uncomfortable terrain located just beyond our current abilities, where our reach exceeds our grasp. Deep practice is not simply about struggling; it's about seeking a particular struggle, which involves a cycle of distinct actions.
 Daniel Coyle

Quotes from the same author

Deep practice is built on a paradox: struggling in certain targeted ways-operating at the edges of your ability, where you make mistakes-makes you smarter. Or to put it in a slightly different way, experiences where you're forced to slow down, make errors, and correct them-as you would if you were walking up an ice-covered hill, slipping and stumbling as you go-end up making you swift and graceful without your realizing it.
 Daniel Coyle
I discovered when I went all out, when I put 100 percent of my energy into some intense, impossible task - when my heart was jack-hammering, when lactic acid was sizzling through my muscles - that's when I felt good, normal, balanced.
 Daniel Coyle
Deep practice feels a bit like exploring a dark and unfamiliar room. You start slowly, you bump into furniture, stop, think, and start again. Slowly, and a little painfully, you explore the space over and over, attending to errors, extending your reach into the room a bit farther each time, building a mental map until you can move through it quickly and intuitively.
 Daniel Coyle