In the last few years much has been made of 'safe-ended' endo instruments.
This is an an attempt to add some clarity of thought to the subject.
The short simple answer is that a file without blades cannot cut.
And any portion of the file that has no blades does no cutting.
If we remove the cutting edges from the apical portion of a file it does not cut.
If we have 1.4mm of non-cutting edge on the tip of a file, the apical 1.4mm of the canal does not get larger.
In rotary instruments any attempt to force the next size instrument into thus un-enlarged area will bind the file creating an adverse torque situation.
If that part never gets any larger, then we have two options in instrument design.
If the canal is tapered and we enlarge circumferentially, then the angle of the wall of a canal relative to the non-enlarged apical portion increases.
If the non-cutting tip of a larger file enters the un-enlarged portion as we move circumferentially around the canal, this in effect, bends the tip of the file back and forth, much as we might do in attempting to break a piece of wire by work hardening it.
If we attempt to probe an unprepared canal we are limited to lump-avoidance probing because we can never enlarge the apical portion of the canal and thus cannot move apically into the canal.
As a result we get the impression from our sense of touch that the canal is blocked as we approach the apex.