In essence, the blocking foul rule gives every player the right to the space and time required to come to a stop in a straight line. If contact occurs, it is the fault of the player who “claimed” that space/time later.
This means that chasing D always needs to allow O to slow down without contact, if D is close this usually means that D needs to play with a bit of separation to one side.
On the other hand, if O changes direction into the path of D who was giving O space to slow down, it *can* be a blocking foul if D can’t avoid contact. It still remains D’s job to avoid that contact if possible, so if D is chasing by 3m, say, and watching O, it is reasonable to assume that D can avoid the contact unless O cuts directly into D.