[blykins]

The issues with a hydraulic roller lifter is that:

1. If you lose control of the valvetrain, the lifter will pump up and hang the valve open.
2. If you use a lot of spring pressure, it's possible to overcome the hydraulic pressure behind the plunger and you lose control.

A short travel lifter is not a typical hydraulic lifter. Instead of 1/2-3/4 turn of preload, they get 0 to 1/4 turn. The plunger stays up to the top of the lifter body and they are built/metered to take higher spring pressures.

These are basically "race-oriented" hydraulic roller lifters and will allow the engine to pull revs that a regular lifter wouldn't allow.