Psionics is a source of power that originates from within a creature’s mind, allowing it to augment its physical abilities and affect the minds of other creatures.
~from Unearthed Arcana: Awakened Mystic
I'm trying to figure out what exactly psionics is in D&D 5e. So it's some vague power derived from the creatures' minds. OK. But then how can it "augment [...] physical abilities"? Shouldn't it specifically be things related to the mind?
For instance, Lethal Strike is an ability that is being playtested at the moment for the newly created Mystic of the Order of the Immortal. This ability allows a Mystic to spend up to 5 psi points to add extra damage to an attack. But this isn't psychic damage -- it's damage of the same type as the attack. So where's the mental part coming in?
If the claim is that through mental discipline/ ability a psionic creature can artificially enhance their own bodies (and presumably detriment the bodies/ physical abilities of enemies), then what exactly is the difference between Psionics and Ki (in terms of what users can actually do) -- other than that Psionics is clearly better because it includes some mental abilities as well? Or in fact, what's the difference between Psionics and Magic other than that Magic is clearly better, being inherently vaguer?
What I'm looking for is an in-setting explanation of what Psionics is, including what it can and can't be used to do. And how this compares and contrasts with other power sources like Ki and Magic.