This inflation technique was introduced many years ago by the Amateur Swimming Association as par of its Bronze, Silver and Gold Survival awards. Some of our older ABers may have taken the award.
I taught and examined for the ASA for a long while.
The technique works well in calm water and one inflation would support a swimmer for at least fifteen minutes before re inflation was required. Rough water conditions would make the technique very difficult. Also, some trouser materials worked better than others but tests were normally conducted in pyjamas !