I was involved in an interesting discussion a couple of weeks ago, which has prompted my to ask a question of the cabin staff on my last four flights.
As they are walking back from the safety demo (or immediately after one of the "non safety" videos which are all the rage these days - don't get me started!) I very politely ask them to demonstrate how to don the life vest while seated and belted in an economy seat or should I stand in the aisle as they did to don it during the demonstration. On each occasion I initially got a bemused look following which two of them admitted they had no idea which option was correct.
Give it a try yourself next trip and let me know the result. I'd be very interested in any "official" response, including how a 120kg person is going to be able to "reach down and retrieve the life vest from under your seat."
Gne