In the example you state, a trill on D and E, you would place your thumb on F and then slide your third finger over D and pluck E, followed by sliding your 2nd finger over D and then plucking E, etc. It feels incredibly awkward at first and I haven’t practiced it enough to be able to do it. But Xavier did it at my harp in my living room with me sitting 3 feet from him, and it was even, clear, and fast. I would never have known that he was executing it that way if I hadn’t seen it.