The biggest enemy to good shifting (unless you're using Di2) is chain tension, or more specifically too much tension. Smooth shifting relies on the chain being able to easily shift off it's current cog or chain ring and this is not possible when the chain is placed under extreme loads, such as when climbing a hill at low cadence.
One problem that many riders face when approaching a hill, in the big chainring, is how to smoothly transition to the small chain ring and maintain an efficient cadence. The usual outcomes are that the rider shifts back to the small chainring too early and then has to rev the last bit before the climb (wasting valuable energy) or they wait too long to shift and then find themselves pushing the big ring and the 23T (or equivalent). In the later case the rider is in great danger of dropping their chain.
So what's the solution? It’s called the double-shift
What is a 'double-shift'?
To put it quite simply it is a rapid shifting of both the front and rear dérailleurs at almost the same time and if done correctly you will barely notice any difference in pedalling resistance nor any changes in speed.
The most important aspect of a successful double-shift is the order in which the gears are changed. In order to understand this is it important that you realise that for any change of gears to occur the chain must be engaged with either the chain ring or a rear cog. Trying to change both front and rear gears at exactly the same time will usually result in slow, sloppy changes and possible a dropped chain. So your first step in making a successful double-shift is to shift the REAR dérailleur down two gears in quick succession (that’s two cogs harder than you are currently using) and then immediately shift your FRONT dérailleur to the small chainring. If your drive train is tuned properly you shouldn't have to wait until the chain has fully engaged on the rear cogs, you should be able to do your front shift the moment you have finished shifting your rear dérailleur. Shifting at the rear usually occurs quicker than the front so by shifting at the back first you can speed up the shifting action and minimise speed loses or changes to cadence.
Timing is another important aspect for the double-shift and it places an emphasis on having an awareness of what gear you are in. As a climb starts rider normally start shifting back through the gears to make it easier. The mistake is made when you go all the way to the easiest gear rather than shifting to the small ring ‘mid cassette’. Ideally you use a double-shift when you are just above half way through the cassette, somewhere around the 16T-19T cog, although it does depend on how quickly the slope increases.
If you are lucky enough to be using Campy Ergo shifters then the double shift is made much easier as the shifting mechanisms lend themselves well to this type of shifting action. For riders on Shimano shifters is a little trickier bit it can still be done with a little more practise. SRAM double-tap on the other hand is a little more problematic for this type of shifting as it relies on the release of the lever to trigger the movement of the dérailleur. This doesn't mean you can't do a double-shift with SRAM, it just means the process will be a little more intricate and the timing between the shifts will be a little longer.
The introduction of the electronic shifting systems, such as Di2 and Campy EPS do make the double shift easier to achieve as the shifting effort is very light. Some systems can even be programmed so that press of the rear shift button for a continuous period results in multiple shifts at the rear. Couple this with faster, more precise front shifting and that’s further improved the quality of the double shift.
As with anything skill, the key to learning and perfecting it is to practise whenever you can. Adding the double shift to your arsenal of cycling skills will not only improve your racing but it will also make your riding more enjoyable and give you that slight edge over your riding buddies on that next climb.