If you remember from Lesson 8: The Manuals, the thumb glissando differs from playing a finger glissando with the thumb--it occurs from white key to white key, or white key to black key.
The Thumb Glissando
Alena Hall shared this technique in a handout:
When the notes are moving inward, the organist plays a white key with the tip of their thumb. While continuing to hold the key down, the organist slides the hand forward in order to continue playing the note with the base of the thumb. The wrist should drop to aid in this process. This frees the tip of the thumb to play the next note, which occurs by lifting the wrist and rocking to the tip of the thumb. When the notes are moving outward, the opposite occurs. The tip of the thumb plays the white key, with the wrist high. In order to play the adjacent note, the wrist drops and plays the key with the base of the thumb.
More detailed information is available to preview here, at Google books.
Here is a visual example:
If you are interested, additional exercises are located on Page 15 of Don Cook's The New LDS Organist supporting materials packet.
A New Hymn
Hymn #11, "What Was Witnessed in the Heavens," contains a few thumb glissandos. Here is an example of one:
Again, I am using Carol Dean's Hymns from the L.D.S. Hymnal Marked for the Organ, mentioned here.
This hymn utilizes direct fingering, redistributing the inner part, finger crossing, finger substitution, finger glissando (the left thumb), thumb glissando, and independent movement. Essentially, every technique that was mentioned in Lesson 8 is found in this hymn.
Using the 15- or 7-Step Method to Learning a Hymn, covered in Lesson 10, learn hymn 11, What Was Witnessed in the Heavens? Practice slowly, never faster than you can play perfectly. Gradually increase the tempo until it can be played at tempo (69-80 bpm).
Read through the text and ensure that your breathing is properly placed.
Get a feel for the mood of the hymn, and choose effective registration for each verse. Practice flawless registration changes between verses.
With this lesson, basic organ technique has now been taught. In the next lesson we'll break away from Carol Dean's book and start learning how to figure out our own hymn markings. Continue on to Lesson 14: Marking a Hymn, Part 1.