What exactly is the effect of channelLastN?

just to clearly understand it please ?

channelLastN: -1 = show all participant videos
channelLastN: 4 = show videos of last 4 active speakers only although everyone has video enabled
… and so on

mean : sound detection + video active = show vidéo ?

After sound end = black screen or last image ?

how can we show last image instead of black screen?