Originally Posted by carpediemracing

With most cassettes having ramps in the cogs it doesn't matter where you shift relative to pedal position. It's virtually impossible to "line up" your shifting with the ramps in the cogs.

Combined with the fact that the chain will shift even under heavy pressure at the ramp this negates the need to worry about shifting at a particular time.

Chains for the last 20 years have mostly been designed with hardened pins especially to handle shifting under load. If your chain has a special installation pin or you use one of the modern master links then your chain is designed to handle the heavy shifting pressure. This includes virtually all multispeed chains sold in bike shops for the last 15 years or so.

With a properly installed chain, i.e. one that won't break under pressure, you can shift the rear derailleur at any point in the pedal stroke, at full power. I routinely shift at the beginning of the power stroke when accelerating hard or while climbing out of the saddle.

(Disclaimer - if you have an older bike, say before the early 90s, you may not have ramps on your cogs. If this is the case then you may not have a chain that has hardened pins. If this is the case then you need to ease up the pressure on the pedals when you shift otherwise you may break your chain.)

The only time you really need to worry about reducing pedal pressure is shifting the front when under pressure, like if you're climbing a hill or you're on a trainer. Then you should ease up on the pedals when shifting. The larger the gap between the chainrings (so this applies more to a compact) the worse the front derailleur will shift. The lower your cadence the worse it'll shift. And the more pressure you have the worse it'll shift.

It's theoretically possible to put so much pressure on the cranks that the front derailleur's spring can't move the cage over from the large to small ring, even with the usual twice-per-revolution ramps built into the outer chainring. If you can shift your front derailleur on a trainer at 5 mph either direction then you've mastered shifting it. I've seen novices on trainers unable to shift the front derailleur on a trainer. I've been in races where a guy tries to shift to the smaller ring while under pressure and the derailleur simply doesn't move.