ESL is the Eastern Soaring League. www.flyesl.org
I edited the first note to spell it out, thanks for asking.
Think NBA or NFL - it is a competition league. If you visit the web site you can read about it.
This particular competition is for gliders that don't have motors. These are launched with a winch. You can see examples here:
That is me narrating the video. it was shot at our field during the June ESL contest last year.
In April we hosted an ESL ALES, altitude limited electric soaring, contest at our field. That is the altitude sensing electric cutoff event you mentioned.