A video the wall cannot play, ProRes or DNxHD, or would play silent, AC-3, E-AC-3, DTS or TrueHD, shows its warning on the row and a Prepare for the wall key beside it.
The key reads the file and plans the one conversion that helps. An opaque picture becomes H.264 in MP4; a transparent one becomes WebM VP9 with its alpha kept; sound that would be silent becomes AAC; whatever already plays is copied, not re-encoded, so a film with a good picture and the wrong sound keeps its picture byte for byte.
Progress shows on the row. The result takes the file's place in the running order, and the original stays where it was with the new file beside it, named film (for the wall).mp4.
It runs on the ffmpeg this Mac has. Without one, the key says the one command that installs it. Nothing is bundled, on purpose: the ffmpeg most Macs carry is built under a licence nothing Sialk ships may include, and running a program the room's own Mac installed is a different thing from distributing it.