How do I boost only the current spin without changing the speed profile?

There is no boost-only call. skipSpin() slams the current drop, and as a round-scoped side effect switches the active profile to the fastest registered one and emits skip:boosted. That speed change only affects subsequent spins -- mid-spin speed switching is not supported -- and the next spin() restores your original profile. For a slam with no speed side effect, use slamStop().