The volume and tone of uilleann bottom D is certainly a function of being played "off the knee," but perhaps more significantly, it is a function of which bottom D technique the piper chooses to play:
- Soft bottom D
- Hard bottom D
Both are played with the chanter lifted, as the sound passes through the bottom hole.
However, it is arguably more than a stylistic difference, insofar as the fundamental pitch of these two notes can differ by as much as a quarter-tone or more.
At a normal bag pressure simply lifting the chanter would produce soft bottom D. This is typically somewhat flat of concert D, usually by some fraction of a semitone. If your drones, regulators, tone holes, etc. are tuned to a concert pitch, then depending on the set, a soft bottom D in the context of a tune can sound jarring and outright wrong.
Hard bottom D is played by increasing the pressure used to play the note a little bit. When the chanter is lifted, you then quickly play an A grace note and then seal it. (This is called a "cut.") If the pressure is sufficient and the cut fast enough the chanter will actually begin to sound a noticeably different sound, which has some almost foghorn-like overtones. This is what you noticed in the Titanic OST. Hard bottom D is an iconic uilleann sound and it is typically more in tune with concert D.
For this reason many pipers will choose to play hard bottom D exclusively, to sound the most in tune, most of the time. As the lowest and in some ways loudest note on the pipes, hard low D is often used as a percussive tool. It can really break up phrases in jigs and reels, and can be cut or otherwise modified with almost any gracenote to great rhythmic effect.
I should add that hard bottom D is also in some sense harder to play. Without focusing on maintaining correct hard D bag pressure, it is easy for a hard D to fall back into a soft bottom D, which can sound very jarring as the flattening of the note will be marked. There's a wide range of pressures that will produce a soft bottom D, but only a narrow band of pressure space that will create a correct hard bottom D without any squeals. Staying in this pocket takes practice.
There are more advanced techniques with uilleann pipes where "bursts" of volume of this sort can be managed, together with lifting or lowering the chanter, to create various effects. For a list of such techniques, check out https://www.uilleannobsession.com/faq.html#cut
For more on the harmonics of hard vs soft bottom D, check out this excellent summary: http://blog.robertrueger.de/?p=157