On X

• open two files to compare in two separate windows
• M-x ediff-buffers RET RET RET
• now the cursor is in a new small ediff frame
• C-h k w d Now I see a help window explaining that wd runs ediff-buffers and how it works.

Maybe you had the cursor in the main frame instead of the small ediff frame?

On terminal

Same as above, but instead of a new small frame I have a new small (one-line) ediff window. With the cursor on the new window, I can ask for help on wd.

Maybe you had the cursor in one of the two windows containing the files instead of the small ediff window?

On X

• open two files to compare in two separate windows
• M-x ediff-buffers RET RET RET
• now the cursor is in a new small ediff frame
• C-h k w d Now I see a help window explaining that w d runs ediff-buffers and how it works.

Maybe you had the cursor in the main frame instead of the small ediff frame?

On terminal

Same as above, but instead of a new small frame I have a new small (one-line) ediff window. With the cursor on the new window, I can ask for help on w d.

Maybe you had the cursor in one of the two windows containing the files instead of the small ediff window?

Maybe things have changed since the question was asked. As far as I can see now:

• the mentioned manual mentions function names, not keybindings only
• there is no command in the manual which is bound to X

On X

• open two files to compare in two separate windows
• M-x ediff-buffers RET RET RET
• now the cursor is in a new small ediff frame
• C-h k w d Now I see a help window explaining that wd runs ediff-buffers and how it works.

Maybe you had the cursor in the main frame instead of the small ediff frame?

On terminal

Same as above, but instead of a new small frame I have a new small (one-line) ediff window. With the cursor on the new window, I can ask for help on wd.

Maybe you had the cursor in one of the two windows containing the files instead of the small ediff window?