To create a different feel with the hammer-on, it can be played faster. Compare the following.
A quick hammer-on uses what is called a grace note. In tab notation the grace note is placed in brackets before the hammered on note. In traditional music notation the grace note is smaller in size with a line through the stem. The grace note is played just before the beat of the hammered-on note.
In the following examples the quick hammer-on is applied to the quarter note triplet. Play the A note immediately after the G note. Use the first and third fingers of your left hand.