Comment (by Zach Etienne):

When Slow Start Lapse (SSL) is disabled, setting initial_lapse = psi^n with n = -2 results in evolutions that are nearly indistinguishable from initial_lapse = W. However, keeping the default as initial_lapse = W is safer for future compatibility: if SSL is later enabled and the lapse is not initially W, you’ll see a spike in the Einstein constraints.

Here’s why: SSL exponentially relaxes the lapse toward *W* very early in the wormhole-to-trumpet transition, smoothing the sharp outgoing-lapse feature that accounts for >99.9% of constraint violations in moving-puncture BBH evolutions (see https://arxiv.org/abs/2404.01137). If the lapse isn’t initially set to W, then “Slow Start” would be better termed “Fast and Reckless”, as the initial lapse will be exponentially driven to W from whatever it was set to initially – leading to the aforementioned spike in Einstein constraints.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2887/twopunctures-does-not-support-slow-start