HR Dynamic Source Switching???

Many thanks! If wearing a HRM is auto selection the best solution?

I have mine on "off". If I can't trust my strap what can I trust :-)

The "best" setting depends a lot on your HRM reliability, wrist HR reliability and whether you wear your watch directly on our wrist or over the sleeve. If your wrist HR is generally ok, leaving source switching on is ok, since occasionally there may be strap HR dropouts or problems (I've had them). On the other hand, if you wrist HR is generally garbage, there's no point in allowing switching to that. And if you wear the watch over the sleeve, definitely turn source switching off, because then wrist HR is turned off when HRM is connected, and that allows wrist gestures to work also when watch is over the sleeve.

Also, the exact behaviour of dynamic source switching depends on the HRM strap used. For Garmin HRM-Fit, HRM-Pro, or HRM-Pro Plus, switching is based also on the measured quality of strap HR, for others it's used only if the connection to the strap is lost.

Your points are good, but I use the HRM (wet the contacts) and I have no problems. One time I forgot my HRM was on the street and thought, it's just a base run. After 10mins the HR was so crap it was unbelievable! Therefore I'm trusting the HRM to 100%.