Here is what Jerry Byrd says about slants, forward AND reverse, in his 1960 Steel Guitar Course (he used the capital letters, underlined):
<b>...the arm or wrist <u>MUST NOT BE TURNED</u>. All action is done by the fingers.</b>
Regarding the "reverse slant", Jerry says:
<b>...it is so important that the fingers do all the work in slanting, instead of the wrist, for if the wrist were used it would be impossible for you to make an accurate "reverse" slant.</b>
Jerry then details 13 steps to the reverse slant... how the thumb slides to the back of the bar, how the first finger asserts pressure on the bar, etc. He devotes two pages to explaining how the fingers are re-positioned on the bar and how to execute the reverse slant.
It doesn’t get much more authoritative than Jerry Byrd! Of course, Jerry used a round bar, not the slotted type. I humbly concur with Jerry, and I too use the fingers for slants (with a round bar)... never the wrist. And I 'correct' my students when I see them turning the wrist to slant. The fingers allow for smoother action, smoother vibrato and faster movement, in my opinion.
Jerry hated slotted bars. He once wrote that the Stevens bar is "an absolute piece of junk".

I tend to agree with him.