How does spacing in practice relate to the spacing effect, and how should you space?

It's the macro, practical version of "space it out": replace one long cram with several shorter sessions on different days, and always revisit old material (cumulative review).

The spacing effect says distributed study beats massed study for the same total time. "Spacing in practice" is how you actually run your week around it. Concretely: swap a single 3-hour cram for three 1-hour sessions on different days, and don't treat old topics as done — fold them back into later sessions (cumulative review). Each return happens after some forgetting, making retrieval effortful and therefore durable. It is the same storage-vs-retrieval mechanism, applied to your calendar.