I've heard those "fixed bar tricep extensions" call push off's of push backs, but never by the fixed bar title (learn something every day). Again, one exercises can go by different names.
The beauty of the movement is that you can adjust the resistance from very light or very heavy by making slight body shifts. Going from a one rep max to a lighter super pump very quickly, all in the same set. Included is a outstanding stretch. Usually the grip is close, as in regular triceps work, than shown in the drawing. The exercise can be preformed just about any where (table, counter, heavy chair, etc..even on a saw horse, which a guy I knew used in his home gym, though mostly for dips). Can also treat this movement like a SC'er or lowering to the upper chest for a different angle on the triceps.
Some will try it for a while but are so programmed to thinking that they need to be using some sort of weight (BB/DB/Cable) resistance, rather than only bwt they never give the exercise a fair shot. Might use it for a light pump at the end of an triceps work, but that's all. Limit thinking can give limited results as far as muscle size goes. The "tri bar ext" can be a stand alone exercise. And produce a super pump, if that is the main goal. This is a pure BB'ing movement.
Might note , that you can shift the majority of the body resistance to one arm at a time, giving more focus to triceps themselves. Good Luck.
Side Bar: Feel free to lower the bar for more of an advanced method, as you progress from the usually waist to lower chest height. If you can do them from a prone (face down) position your getting pretty good.