Hold finger at 1/2, pluck with finger at 1/3: octave up
Hold finger at 1/3, pluck with finger at 1/2: fifth up
Remove one finger, pluck with other: tonic
You can sound notes as quickly as you can get a raised finger back on the string.
Of course, both hands are fully occupied with one string, producing three notes, instead of being occupied with six strings producing six notes. That's why it's a Stupid Lyre Trick.