Watch the Grace Note Hammer-On online guitar lesson by Chris Buono from 60 Electric Guitar Techniques You MUST Know
Like hammer-ons, grace note hammer-ons or hammers are an ascending technique that call for a fret hand finger to activate a note by way of increased initial fretting pressure sans picking of any kind. What sets it apart is the phrasing. Grace note hammer-ons come almost immediately after the picked note, hence the grace note.
Looking for inspiration? Grace note hammers are a huge part of Jimi Hendrix phrasing when he would play rhythm parts in signature tunes like "Wind Cries Mary" and "Little Wing". It's a go-to phrasing tool for all clean tone leaning scenarios across all styles.