The Transfer Method (also called ‘méthode transfert’ in French) - the technique was developed in Germany in the 1940’s. The process follows the first steps of ‘Méthode Champenoise’ (Traditional Méthode) - in that after primary-first fermentation of the cuvée (blend) is assembled and transferred into bottle to complete the secondary fermentation.
When the secondary fermentation is complete and the wine has spent the desired amount of time in bottle on yeast lees (six months is the requirement to label a wine 'bottle fermented', though some producers only wait 90 days) then the individual bottles are transferred (hence the name) into a large pressurized stainless steel tank - where it is filtered, removing the yeasty sediment. Unlike in Traditional Méthode - no riddling and disgorging used.

 

The wine is then filtered, the liqueur de expedition (also known as dosage) is added in bulk, and then bottled under pressure. The final result is fairly indistinguishable - and then filled back into new (perfectly clean) bottles for release on the market.
This method is a less expensive alternative to the time-consuming and extremely involved manual process used in the traditional méthode process. It also allows for complexity to be developed into the wine, but also gives scope for blending options after the wine has gone into the bottle and reduces the bottle-to-bottle variations that can be hard to control in the ‘Traditional Champenoise Méthode’. Though there will always be detractors who feel that the process strips fine flavour elements, especially key yeast flavours.
Even though it is said to be a less expensive process when compared to the Traditional Méthode. It must be noted that the set up costs can be inhibitive for many - as the machinery (i.e. the specifically designed pressurized tanks) are expensive.
A bottle label or tasting note stating - ‘Fermented in this bottle’ means Méthode Champenoise / Traditionnelle - whereas ‘Fermented in the bottle’ refers to the transfer process; so I encourage you to read the fine print on the label.
Champagne (Méthode Traditionnelle producers) generally use the transfer method to produce any size bottle smaller than 750mls or larger than 3 litre, plus limited edition bottle shapes and sizes - though there are several top Champagne Houses which bottle ferment in 3 litre bottles and even larger - (e.g. Melchizedek - 30 litres = 40 bottles; by Drappier).