a technique where two notes are heard but only the first note is played by the picking hand
Hamer-on is a technique used in guitar playing.
A note sounded literally by "hammering" down with a left hand finger, often performed in conjunction with a note first plucked by the right hand on the same string.
Hammer-on is a stringed instrument playing technique performed (especially on guitar) by sharply bringing a fretting-hand finger down on the fingerboard behind a fret, causing a note to sound. This technique is the opposite of the pull-off. Passages in which a large proportion of the notes are performed as hammer-ons and pull-offs instead of being plucked or picked in the normal fashion are known in classical guitar terminology as legato phrases.