I tend to agree, Jamt, the terms don't seem to lend clarity.
I was playing with this a little on my run down after skinning this morning. I guess I feel what I understand to be crossunder to be more along the lines of different trajectories and maybe more a decoupling of the lateral connection/component.
I suppose I understand the desire to relate this to unweighting and maybe it technically is. Though, I think of unweighting more concerning the connection between the bos and the snow, and the vertical piece. I might be more inclined to think of this as float vs unweighting, but that is just my distinction and likely doesn't lend clarity either, but seems more fitting for a more dynamic turn where things have ramped up to the catch and toss mode where we can work off the impulse from the ski to define our trajectory and not just in opposition to or management of gravity.
To clarify I might suggest that a crossunder be a decoupling between the bos and com while maintaining the connection and interaction between the ski and the snow. So since the skis are still engaged and working when the resistance of the com is removed they seemingly fire across underneath and this gives rise to the feeling of the feet/skis crossing under.
Is it necessary to "unweight" here? I don't think so. I think we could do "retraction" turns attempting to keep our com stable with skis arcing back and forth in an attempt to maintain constant pressure under foot throughout the turns and feel this "crossunder". Likewise, I think we can ramp it up to dynamic turns reaching across the hill with a float of the com and still keep the skis interaction working to the extent necessary to get the skis to zing across.
Whatever, have fun experimenting with it