I virtually always shift in sprints, otherwise I'm not accelerating that much. I usually shift as I jump, then one more shift during the sprint. If it's a slow sprint (like uphill) I may shift more times.

I try to shift at the top/bottom of the pedal stroke (it's kind of instinctive now), full power, but at those points in the stroke "full power" is not that high. However I regularly end up shifting in the heart of the downstroke. It's all good, with the ramped cogs and modern chains (hardened pins). I used to shift under 100% with toe clips + straps, a soft Sedis chain, and unramped cogs. I'd pull my shoe out of my pedal even with 2 toe straps cranked so hard that my feet hurt. With 3 straps I only pulled out once, but that was about when someone invented clipless pedals.

A chain that fails is a chain that shouldn't be on the bike in the first place, either due to wear or improper installation. I've seen many new chains fail at the spring series at Bethel, new ones installed improperly.

cdr