Phil,
Yes, the preserver in the belt pack is to be pulled over the head once inflated.
That said, I typically wear/bring an internal frame daypack with food & other safety gear, so I've got those straps over my shoulders. I also use a wading staff.
I disconnect the daypack belt snap before any crossings. Should something go sideways and I go for a swim, my first goal is to get to or stay on the surface...inflating the preserver, if necessary, and dumping the daypack.
Depending on what I'm faced with, I'll either get the preserver over my head or make my way to safety, while on my back, using the bouyancy from the inflated preserver at my waist.
My hopes/goals are:
1) When crossing, to choose a safe spot in the first place, so I don't run into problems and need the preserver.
2) To turn around if the crossing point was misjudged and I can safely do that.
3) To pick a crossing point that, should I go for a swim, I have a sufficient distance before any hazards to get to safety and/or activate the preserver, get it over my head, if necessary, before heading for safety.
Of course, when shit happens, it often happens fast. Whether or not I'll be able to get it over my head is an unknown if I'm tumbling down stream...but I figure the bouyancy at the waist is better than what I had before...which was nothing...aside from the flatulence that filled my waders as I crapped my drawers when going in.
I test the unit once a year or so and inspect it often to make sure there's no abrasions/tears whatever. I have tested getting it over my head, which was straight forward, but this was not in moving water.
Mike d