I was glancing through a potential student's exercise book and noticed a comment he'd added on a page of rock rhythms: “I can play these easy.” However, from watching him play it was clear that he could not. So what gave him the impression he could? Likely he was able to play them a little bit ... at one time ... in isolation ... alone in the practice room.
When you're working through a group of exercises, it's not uncommon to be able to breeze through a
few of the more advanced ones after a good warm-up on related items. To me this amounts to plain beginner's luck, and it's too easy to conclude that you’ve reached some major milestone. Yes, you did play them, and probably well enough ... for a few bars. But they won't have become part of your vocabulary. That takes a lot more than a few lucky executions.
A practice standard I like to use is to play an exercise for a full 5 minutes non-stop. It's a practical length of time -- about as long as a song might last. If I can play something for 5 minutes, I'll probably be OK out in the field. But I’ve notice an interesting phenomenon with this approach.
The beginner's luck idea might predict that you'd be able to play a new exercise, more or less, for a few bars at least, and often more. Press on and you'll likely find that things start to fall apart around the 1 minute mark. Struggle onward and you might be OK until around the 4 minute mark, where it begins to fall apart again. And while that last minute may be a challenge, pressing on will be worth it.
I've always been intrigued by the consistency of this phenomenon. Of course I have no explanation for it, but it does suggest that anything practiced for less than a minute -- the beginners luck zone -- doesn't get you anywhere, and even 2 to 3 minutes might not suffice.
The key to all mastery is repetition, repetition, repetition. (It’s also possible that the struggle is an important part of the process.) So don't be misled by beginner's luck. If you can play it for a few bars, that's nice. But to really get it, shoot for 5 minutes daily for at least a week. Then it will truly be yours to keep.