It is difficult to simultaneously a) gain familiarity and skill with a new tool and b) apply that tool well under high-stakes circumstances.
Think of the metaphor of someone learning to lift weights. You do not typically load up the bar with large amounts of weight while learning the proper form for an exercise.
Rather, you use a very lightweight bar and focus on getting all of the motions down correctly, and you practice that until it no longer requires your full and active attention. Then you add weight to the bar.
Similarly, when practicing some new mental algorithm, such as a therapeutic tool that can help you solve a difficult problem, it's wise to first practice that algorithm on a simple, low-stakes problem, so that when you attempt to tackle your real problem, you aren't also struggling to remember how the tool even works, and possibly blaming the tool for what might be a failure of your own skill in applying it.
