Watching the Amazonas5 launch, and at 1st stage separation, the commentator noted that the 2nd stage would ignite before the separation in order to "maintain constant acceleration."
I'm wondering how this works - surely the 1st stage is still in the way, so the rocket will be pointed directly at the top of the 1st stage.
Alternatively, if this is beneficial to the launch in some way, why don't all launches do this?
Why do the Russians use these fence-like interstage fairings? is related, but not really a duplicate.