For those that watched the second launch of RocketLab's Electron launch vehicle, out of Mahia Peninsula, New Zealand on 21 January 2018; in which the vehicle successfully made it to orbit for the first time, this curious sequence of events occurred approximately four minutes into stage two burn.
Operators first called out a number of updates, before two items physically detached from the aft-end of the second stage of the vehicle.
T+6:26: "Coming up on hot swap".
T+6:38: Two items detach and fall away from the vehicle
This can be seen in this frame captured from the live broadcast of the launch on YouTube. In it, one of the items that fell away can be seen to the left, just below the Rutherford engine bell.
What are these two items falling away from the vehicle, and what is "hot swapping"?