I just watched Kyle’s SubD tricycle video. Several times he selected a section of curve and using the gumball aligned it. In the moment (pic. 2:18:40) it looks like he entered a number – I imagine 0 – but the cursor blocks the box. I’ve had this problem many times and I think a I do a lot more to get what he did.
He probably relocates the gumball, then selects one of its scaling handles, and enters 0.
This technique can be used to flatten or planarize for instance three points, where two are coplanar and one is not. If you select the three points, relocate the gumball to one of the coplanar points, and select the scale handle normal to the relevent point plane, and then enter a scale factor of 0, the third point will be moved to also be coplanar with the other two points.
This technique can of course be used with other geometries, not just points.
He’s put up a short video on straightening edges: https://youtu.be/mCPjdmitzXU