Soft-proofing is a different beast… shortly speaking, the purpose of soft-proofing is to simulate the way an image looks on a given output medium A, by using a different output medium B. Typically, you want to use your display to check how a photo would look like once printed.
Therefore, the ICC profile that you select at the bottom of RT’s preview window should correspond to the output medium you want to simulate, and not to your display device.
Technically speaking, in soft-proofing mode the image is first converted from the working colorspace to the soft-proofed profile (step 1), and then from there to the monitor profile (step 2).
Step 1 is where channel data might be clipped because out-of-gamut, and where you can eventually recover part of this clipping with the perceptual intent if the soft-proofed ICC profile supports it.
Notice that soft-proofing only makes sense if your display covers a gamut that is wider than the one of the soft-proofed profile. Otherwise, there is no difference with respect to going directly from the working profile to the monitor profile.
Notice also that modern ink-jet printers normally can exceed the sRGB gamut at least in some hue ranges, therefore you need a wide-gamut display if you want to get a meaningful soft-proofing of an ink-jet print.
Another aspect that can be emulated via soft-proofing is the black level of the final print. Assuming that the black level of your display is lower than the one of the ink-jet print (blacks are “more black” on your display compared to the final print), you can simulate on-screen the lower contrast of the final print that results from the higher black level.
For this, you have to enable the black-point-compensation (BPC) in step 1 (black in the working colorspace will be mapped onto the darkest color the printer is capable to generate), and disable BPC in step 2, such that the black level of the print will be “mapped” onto a RGB values above (0,0,0) in the data sent to screen.
I hope this has clarified at least some aspects, and not generated even more confusion…