Since I have been a PT, I have implemented Instrument Assisted Soft Tissue Mobilization (IASTM) into my treatment. Originally, I was trained to "dig deeper" like some of the more prominent brands preach, but advances in research have shown our effects to primarily be neurological and more successful with lighter implementation. Sometimes, no matter how light you perform IASTM in certain areas, you will note some redness developing. These are typically highly tonic areas. Normally, that response may be contraindicated to continue IASTM, but there are ways to adjust. Dr. E from The Manual Therapist recently had a post discussing why these areas can be so easily reddened. He also offers several examples of how to address these instances.
Personally, I think these are great tools to implement as I regularly find HS insertions that are significantly tonic and these patients may benefit from these attempts at calming the nervous system. However, I do disagree on "always light" IASTM technique being better. From my experience, I have had a significant number of patients that had previous experiences of different forms of IASTM that are more aggressive with the old school approach. Having tried the lighter technique on these patients, typically they subjectively feel like they don't improve without the more aggressive approach, even if objective results show differently. With what we know about how a patient's beliefs impact their rehabilitation, I tend to be a little more aggressive with my IASTM technique on these patients, although, still not nearly as aggressive as others.