At the past 4 Masters meets I officiated (including 2 Nationals), I have observed slower butterfliers completely submerged at some point in the stroke cycle (after surfacing before the 15 mtr mark). Although the swimmer is not attempting to submerge for the purpose of streamline dolphin kicking, their momentum coupled with slow turn-over takes them completely underwater for short periods of time.
The video link - http://vimeo.com/248356962 - shows me swimming slow fly. For brief moments I am completely submerged with no forward propulsion - just resting before the next stroke.
An overly strict, by-the-rules official would be within the rules to DQ a swimmer doing butterfly like the video. It is called "resubmerging after the 15 mtr mark".
This is one of those cases where Masters might need an interpretation of the rule - specific for masters swimmers - that allows for submerging during a stroke cycle provided the swimmer does not perform more than "x" dolphin kicks while submerged.
