Helt riktig der moepower og som sagt fikk jeg en litt bedre forklaring på engelsk:
First of all, we have to have a quick re-cap on how turbocharging works. Essentially, exhaust gas is used to spin a turbine which compresses intake air. But what happens when you shift gears? You have to lift your foot off the throttle. Exhaust gas velocity drops, and the turbine blades slow down. Intake boost pressure drops. For the next gear, you have to wait until the turbine spins up again for the boost to kick in. This is what you call turbo lag. However, turbo lag should not be confused with boost threshold, which is the minimum engine RPM which is required for the turbine to physically supply boost.