To be cynical, I often felt the sine wave was just ITF's way of differentiating itself from old Chang Hon groups.
I recall when GTI (Global TKD International) split from TAGB (TKD Assoc of Great Britain) in the early 1990's. GTI decided to look at (re)joining ITF. It's worth pointing out that TAGB was formed by some of the top UKTA (ITF) practitioners in UK breaking away. many had studied and graded under Master Rhee Ki Ha and General Choi.
GTI arranged with ITF for a seminar, workshops and discussions about (re)joining ITF. The General came over to England and held a seminar where he outlined the sine-wave. This was something totally new to those of us who weren't ITF. He pointed out how "true TKD" (not "phoney WTF karate").
Considering how many of our instructors had either studied first hand with, or under high ranking students of Master Rhee, I found it strange how in 10 years they could have become such out-of-date practitioners.
Other commentators on these boards who are much more knowledgeable than me have pointed out sine-wave (or elements of it) existed for a long time in original ITF practice before it became as prominent as it is today, so perhaps I am just being a little too cynical.
See how well I block your punches with my jaw!!
Supporting everyone saying "nuts to cancer"