Hey gang, one of my favorite topics…
Regardless of whether one “believes” that all hammer-ons can be done, Holdsworth is on record striving to use the technique. This does not mean that he never uses other techniques, nor does it suggest that he does it all of the time. It does inform the approach he evolved throughout his lifetime. Ultimately his choices in the moment were undoubtedly a question of art and preference and not some book of rules.
(Is the ability to hammer on ascending questioned? Is one secretly pulling off when ascending with hammer ons? Is two hand tapping a la Stanley Jordan et al, impossible? Suffice it to say the evidence of all hammered lines are numerous.)
Getting the optimum “dance” on the strings is an exercise in subtlety, but IMHO, well worth it. The side benefits to the practice, such as increased finger speed, forced attention to connecting notes, and relaxation while playing, are immeasurable in their value.
How many tend to grip with their index finger? How quickly does one get over that bad habit while pulling off in practicing all hammer ons? Hammering suggests an approach that expands the variety of pull off textures possible.
Gabel, Harrison, Quayle, Graham… All these folks suggest fantastic exercises and hints that take the mystery out of the approach, so I second the recommendations.