The Denk power switches are all basically the same regardless of vintage. The "logo" slide switch arm has the Denk logo imprinted on it. It has a reducer element in it, ~.66X. The other switch, called the high power arm, has a barlow element in it. The OCA is actually a barlow lens too with an amplification of around 1.8X by itself.
For an SCT, you a couple power switch choices in configuration, basically with and without the OCA.
Without the OCA you just screw the power switch into the bottom of the viewer and then the 2" nose piece into the bottom of the switch. That whole assembly is then placed in the 2" star diagonal. When you push in the "logo" slide switch in, you get ~ .66X. You can get a focal reduction and still be able to reach focus because most SCTs and Maks have a very large range of available back focus because they focus by moving the primary mirror. This movement is amplified by the power of the secondary mirror. The advantage of focal reduction is the lower power. However that usually comes at the price of clipping the aperture of the scope a little.
When the switch arms are pulled out, there is no extra element in the optical path so the beam from the scope goes directly into the viewer. This actually results in about a 1.1X amplification as you need to refocus the scope (over that of single eyepiece use) to make up for the extra 5" of optical path length a typical viewer has.
With the high power arm is pushed in, you ~2.0X amplification, and, again, you have to refocus.
With the OCA, you simply screw the OCA into the bottom of the 2" nose piece with the configuration above, then, just like above, insert it into the 2" star diagonal. Now since you are stacking the OCA element (which is a barlow) with the other elements of the switch arms, you will get different magnifications. Specifically ~1.4x with the low power switch arm pushed in, 1.9X with both switch arms pulled out and 3.0x with the high power switch arm pushed in. This is also the same configuration you will be using with a typical refractor with a 2" diagonal, however, the magnifications will be slightly different (~1.3X, 1.8X, and 2.6X) as you are not shoving a primary mirror around to focus. The low power, 1.3X, mode will require a bit of focuser in-travel relative where you would focus with a single eyepiece, the 1.8X about the same, and the 2.6X mode will require you to run the focuser out a bit.
With a newtonian, you use the same system as the refractor system except you need to add an adjustable spacer tube between the 2" nose piece and OCA because almost all newts just don't have the range of focus you get with a refractor and you need to push the focus plane out a bit more from the side of the tube as a result. You will also typically need to adjust the spacer tube length to fit the range of focuser travel that's unique to your newt. But this is easy to do really. Magnification factors will be a little higher than those for a refractor.
I hope this helps.