Teaching students how to play extensions is something that I struggled with for many years. Part of the confusion was that there are different ways of playing a whole step between fingers 1 and 2.
Here are the big differences:
- To play a whole step between fingers 1 and 2, you can play (at slower speeds) by sliding your thumb along the back of the neck, towards the finger that will play the new note. (slide thumb towards 2nd finger if you're coming from 1st finger or vice versa) Once the student can play with vibrato, this way of playing extensions will happen naturally since the thumb must always be situated in a balanced place for vibrato.
- Another way you need to learn to play extensions, is for when the music is at fast speeds, or when the hand has to play a double stop. This is the "double stop method" or "tunnel method"
I teach the "Backward" extension first which you can see in the video below and then usually wait at least a few weeks before teaching the "Forward" Extension.
I have learned that it takes a long time for extensions to settle in and become automatic in a student's playing. You need to keep trying backward and forward extensions in different pieces and etudes. There are some wonderful extension etudes in 25 Etudes by Rudolf Matz. I especially love #17.
The first part of the video below is the end of a lesson where I had just taught a student how to do a backward extension. From 1:00 to 1:45, I show a great trick from Cornelia Watkins and also how NOT to do a backward extension.