Does that mean the return man will make more of an effort to avoid oncoming tacklers or does that mean he will simply break more tackles?

I've always taken it to be Bort's way of combining both fakes and broken tackles into one phrase, tackle avoidance. But that is just my interpretation, so you may want to wait and see if anyone else has a more definite answer (i.e. a Bort quote on it).

No quote but tackle avoidance is basically just that. When your returner and a tackler meet there is a roll to see whether you get tackled or if you break the tackle. Tackle avoidance would increase your chance of not being tackled. It won't make him more elusive or jukey or whatever. Just makes it harder for someone to tackle the ball carrier.

It's break tackle. Check out the text for stiff arm.

i dont have the quote (im sure perhaps someone else does though) but bort has confirmed that by tackle avoidance, he means break tackle.