Extensions emphasize the long head much more than any other triceps exercise, leaving them with the best potential for size gains since the long head is the largest part of the triceps.
Yes.
And to further support this, as well as the notion (also frequently suggested by Pumpster) that extensions behind the head are superior builders of the long head, read this article by a former "Mr. O.”
http://www.larryscott.com/bio/newsletter/97summer_2.cfmI’ve been doing these as described for about a month now, and they are very forgiving on the elbows, the stretch is great, and the soreness (good PW kind) the entire length of the triceps is unequalled.
If it is inconvenient for whatever reason to use this setup, consider doing bodyweight tricep extensions (a.k.a. pushbacks, pushoffs, tiger extensions, etc.), which provide a similar feel.
Someone was just talking about those on here the other day, though I can’t find the particular thread.
I believe a thorough description appears in the Arnold Encyclopedia for those who are interested and ambitious.