I think you balance the two tricks by defining "Figure out enough to get started" as "Understanding the problem (without overthinking it)."

For me this means walking through use-cases of an idea "on paper" while resisting the temptation to actually design the solution. Once you've defined the verbs/nouns of the system you're much better prepared to "dive in" and build something. That, in turn, will uncover things your paper system forgot or burst your illusions about certain use-cases causing you to rethink them. Rinse, repeat.