Hammer-ons and Pull-offs are advanced techniques used in the Guitar Hero series of games. Many fans know these by their common abbreviations, HOs and POs (or HOPOs). Although labeled as advanced per se, it is relatively easy to learn how to perform the technique, but to master it is a totally different story. Indeed, in order to perform the insane solos that the game sends at you, HOs and POs are not only recommended, they are essential.
When a close set of notes is approaching on-screen, rather than strumming each note individually as normal, you hit the strummer for the first note then finger the other frets, but you do not have to strum all the notes; the notes still "play" as usual. To "hammer-on" means to perform this technique starting out with a lower note, and usually ending the note sequence in a higher note. An example would be a three note sequence of green, red, yellow. You would strum the green note as normal, but use the above technique to "nail" the red and yellow notes or "hammer-on" them. To "pull-off" is to use the same technique once again, but in reverse. Usually, a pull-off begins with a high note, and ends in a lower note. An example would be a three note sequence of blue, yellow, red. By strumming the blue note as normal, you "pull-off" from it and finger the lower notes in the sequence.
Once learned, and eventually mastered, the techniques greatly reduce difficulty later on in the game, especially in Expert mode.
When to Use
Early on in the game, HO's and PO's are not able to be performed simply due to the fact that notes in a sequence must be "close" together on the fret board; the technique can not be used on notes too far apart. Therefore, because of the slower speed of Easy and Medium, opportunities to use HO's and PO's are almost nonexistent, although the ending of Sweet Home Alabama for the Bass in Guitar Hero World Tour features numerous HO's and PO's even on easy. However, when playing songs on Hard and Expert, HO and PO sequences are often in abundance, and as stated before, are many times essential to nail insane solos; to strum each note in fast sequences and solos seen often in Expert would be tedious and reduce the likelihood of hitting the majority of the notes; in rarer instances, it is downright improbable to be able to strum each note individually.
HO and PO sequences vary in length; some can be simple two note patterns, such as in the intro of "Billion Dollar Babies" on the Xbox 360 version of Guitar Hero II; others can be six, seven, even ten notes or more. A very skilled player can often hit dozens of notes coming down the fret board using solely HO's and PO's. Some songs, such as "Jordan" on Expert are indeed impossible to pass without skill in performing these techniques. Each player often is able to see what works for them, growing comfortable with how often and when they use HO's and PO's.
Mechanics and Technicalities
HO and PO sequences are handled differently in Guitar Hero compared with the rest of the series - they play closer to the way they would on a real guitar, making them more realistic but far harder.
Consider a sequence green-red-yellow. In the first Guitar Hero, hammering on this sequence would require you to strum the green, then press the red while still holding the green, then press the yellow while still holding at least the red and most likely also the green. In all other Guitar Hero games after the first game, you can strum the green, then release it and press the red, then release that and press the yellow. Conversely, a yellow-red-green sequence would need to be played as a strum with yellow, red and green all held, then release the yellow, then release the red - in later versions you can simply press the yellow, then press the red, then press the green. This has considerable consequences - it's these differences that make Bark At The Moon so hard on Guitar Hero, and also renders tapping virtually impossible.
A trick to somewhat fix this in a long sequence is to strum the first few notes and hammer-on the rest. This makes the timing window larger somehow.