How well do you know what mile race pace feels like?
What if I took away your watch?
What if I took away the track?
Applying constraints challenges the extent to which we've mastered something. If you've really mastered it, the external conditions do not matter.
Let's venture into the world of drum corps for an example.
For roughly 15 minutes, a corps performs a musical arrangement in a visually appealing spectacle on a football field. Simply being able to play the music from memory is only a portion of what's required to execute a good show. Corps are constantly moving, creating shapes and using props to enhance the story told by the music. Thus, each individual member must be a master of their harmonic contribution.
Watch the opener of Phantom Regiment's 2010 show "Into the Light." Familiarize yourself with the sounds coming from the horns starting at the 1:50 mark.
Now, with that melody in your head, watch this next video. The horn section was asked to play that portion of the show in practice. But there was a small catch: they didn't use their instruments.
They sang.
Pretty cool, right?
Constraints create layers of mastery. The greater the constraints, the closer to mastery one must be in order to consistently deliver.