Sandhi is a Sanskrit name for a set of linguistic phenomena involving changes and combinations of tones or consonants. Sandhi is what scared me off studying Sanskrit several years ago (but I shall try again, one day). There are many forms and types of sandhi, and not only in Sanskrit (even the English language's phenomenon of variety in the indefinite article (i.e. 'a' vs. 'an') can be called sandhi).
In Devanagari, the classical script used for Sanskrit and the one I was trying to learn to read, sandhi has a visual aspect, wherein two characters combine, or meld, into one compound character, bearing distinctive elements of both original characters.
Wagner's innovative use of leitmotifs is well-known. My realization is that he sometimes performs sandhi magic with the motifs, melding them ingeniously. It struck when Wotan was scolding Brünnhilde for disobeying him (Act III), and he mentions Valhalla, evoking the regal Valhalla motif, but not as it appears elsewhere (and in particular in Das Rheingold); here it is subdued, almost forlorn, matching the context. This interplay of theme and context, motif and variation, is Wagner doing sandhi.
It is also one reason for my fascination with his works.