Illegal blocking techniques

[kentref]

Two points re your question:

1. Is the blocking action, although technically "illegal" by the book, creating an advantage/disadvantage?

2. Is the blocking action such that it could injure the player being blocked, (i.e., is it a safety issue)?

If you've got either, then maybe you've got a foul. Flagrant use of a forearm, for example, probably rates a personal foul as opposed to a hold/illegal use of hands foul.