B to C# in the First and Second Octaves:
By providing the C# with a full-sized tone hole, the intonation is better. Additionally, the two-finger trill is avoided.
C to C# in the First and Second Octaves:
Moved from the first finger left hand to the first finger right hand, the trill is generally faster and better in tune.
An expanded version from here; “Our mission at Burkart is to enable the flutist to push the performance envelope by giving full voice and limitless technical possibilities to the instrument. Every Burkart flute, piccolo, and headjoint truly embodies the motto ‘The Sound and Feel of Great Design.'”
High G to A in the Third Octave:
Perhaps the better name for the C# trill key would be the High G to A trill key. Finger high G and trill the C# trill and the upper trill key. An otherwise awkward, treacherous, and too-common trill is made easy.
High G to Ab:
Better in tune and much more comfortable.
High F# to G#:
Finger high F# and trill the C# Trill Key. Much easier than the conventional fingering.
High Ab to Bb:
A Mahler favorite. Finger the high Ab and trill both trill keys and the C# Trill Key.
Tremolos and Shakes:
All the first octave, left-hand notes can be tremoloed with C# by just using the C# Trill Key. The same notes can be tremoloed with D natural by using the C# Trill Key and the upper trill key together.
Pianissimo high Ab
Play second octave Ab with all the left hand keys depressed, add the C# Trill and the high Ab will appear softly and in tune.
Other Musical Benefits:
John Wummer once showed Jim Phelan his favorite use for the C# Trill Key. In Debussy’s “l’apres-midi d’une faune”, he would play the opening C# with the conventional fingering (all open) with no vibrato and pianissimo. Developing the phrase, he would return to the C# playing it with the C# Trill Key with vibrato and louder. This illustrates something important; the small C# tone hole, first finger left hand, serves two purposes. It, of course, produces the pitch C#. It also acts as the vent for the production of second and third octave notes that are harmonics of pitches in the first octave. Unfortunately, a vent should be small and a tonehole for a C# should be large. The C# Trill tone hole is exactly a larger tone hole for an in-tune, sonorous C#.
There are several ways to build a C# Trill Key and we have repaired them all. The Burkart Flute is built with two additional support posts, a guide post, a separate steel rod, and spring for the C# Trill. It is our experience that this mechanism is far superior to any other. The C# Trill lever is placed forward of the Bb shake lever and is not in the way, yet still convenient. Through careful comparison, Burkart has determined that the C# Trill tone hole in no way affects any other notes or the intonation and timbre of the flute as a whole.