How Hammer-Ons and Pull-Offs Are Notated in Tab
Hammer-ons and pull-offs allow several notes to be played for every time a string is picked with the right hand. Both techniques are indicated by a curved line (called a slur) above the affected notes.
A hammer-on is played by first playing a note and then playing a second, higher-pitched note by forcefully bringing another finger down on that note to make it sound. Hammer-ons may be indicated with just a slur:
or with a slur and ‘H’ above the slur:
Hammer-on from Nowhere
It is also possible to hammer-on without plucking the string first. This is known as a hammer-on from nowhere. A hammer-on from nowhere is indicated with a small curved line before the note.
A pull-off is played by first playing a note, then pulling the string down slightly as you release the note. This releases the initial note to a lower-pitched fretted note or an open string, making it sound. Pull-offs are indicated with a slur:
Sometimes a ‘P’ may be placed above the slur:
Combining Hammer-ons and Pull-offs
Multiple hammer-ons and pull-offs are possible. This is indicated by placing a curved line over the affected notes:
It is possible to place an ‘H’ or ‘P’ above each hammer-on or pull-off, but this is generally not done because it would clutter the music.
Trills are a rapid alternation between two notes using a combination of hammer-ons and pull-offs. A trill is indicated with the letters ‘tr’ and a wavy line above the trilled notes. The notes to be trilled are shown with a lower note and a higher note in parentheses. Refer to the music notation for the rhythmic duration of the trill.
Learn how to read hammer-ons and pull-offs in tablature and how to interpret the symbols.
Pull-off is a technique when we use a finger to fret a note and sound that note, while the note is still sounding, a finger can be placed behind the first finger, and then the first finger can be ‘pulled off’. By doing this, the effect is the same as plucking the string with a finger, it creates vibrations. And now the note that is being fretted right behind the first finger will be sounded.
Pull-offs create vibrations, while hammer-ons mostly don’t. In other words, in performing a pull-off, it is possible to make the second note sound louder than the first. 
Motives of use
It’s used to make a passage easier to play. This is true for two reasons. The first is that we can usually do a pull-off faster than we can pick two notes in succession. The second reason is that while performing a pull-off with two fingers of the fretting hand, it’s possible to play something else at the same time with other two fingers and pick.
\”How to play pull-off\” guitar lesson
History of pull-off technique
In \”Ellis’s Thorough Course For 5 String Banjo\” written around 1900, the term ‘pull off’ is used to explain the action of performing technique called ‘snap’.
The term pull-off was first defined by Pete Seeger in his book \”How to Play the 5-String Banjo\” in 1962.
Pull-off is a technique when we use a finger to fret a note and sound that note, while the note is still sounding, a finger can be placed behind the first finger, and then the first finger can be 'pulled off'.