What is the terminology for these two techniques on piano?

Question

Disclaimer: I don't know music terminology, as I don't play any instruments.

The channel Sheet Music Boss on YouTube has two videos in which I've heard what I perceive to be two different techniques of some sort being played on a piano. I'd like to know if these techniques have any specific terms so that I can search for more, because it gives me goosebumps.

In the first video titled: IEVAN POLKKA (LOITUMA POLKKA) - Piano Tutorial at about 2:03 to 2:18 the music seem to change character, and becomes, for lack of a better term "magical". It looks to me as if the melody is still being played with the right hand, but something changed with the left I believe.

In the second video titled: When Johnny Comes Marching Home - Piano Tutorial at 0:57 to 1:20 it's pretty clear that the melody is being played with the left hand, while the right hand is playing what I assume would have been the bassline if it had been played with lower notes.

Answer 1

Ievan Polkka

There are two things happening here:

1. In the preceding section, the left hand is playing pairs of notes simultaneously. However, beginning at 2:03, the left hand plays broken chords. Broken chords, also called arpeggios, are when notes forming a single unit (a chord) are played individually.
2. The right hand plays the melody with ornamentation, meaning that decorative notes are added to the basic melody. In this specific case, you can hear occasional faster notes that were not previously heard. This is also sometimes called elaborating the melody. It can also be considered a variation, which is a technique by which a melody is restated in a modified form.

When Johnny Comes Marching Home

In this recording, the right hand plays broken chords (see above), while the left hand plays the melody. This technique of exchanging ("swapping") the roles of the two hands -- or more generally, having two instruments or "voices" change musical roles -- is called voice swapping.