Ok it's obvious that I need to make it clearer, if you watch the video from above, he makes a plane, adds wings at an odd angle and then attaches a sub assembly of intake, fuel tank and engine to the wings.
He then uses the GIZMO's to align the sub assembly to the HANGER grid that the mod displays, at this point he is EYEBALLING the alignment to the hanger grid.
I use Editor Extension Redux, Here's a video on the horizontal/vertical SNAP along with some SNAP functions for struts & fuel lines...
With this mod I can place a part, point at it and hit "V" or "H" to center it horizontally or vertically compared to the parent part.
What I'm suggesting is something similar to that for hanger grid, place a part, point at it and hit the proper key and it will snap to the X, or Y, or ,Z or all 3 of the HANGER or other keys to snap to the X, or Y, or Z of the ROOT part.
Lets say that in that video he had not just offset the sweep of the wings but had also tilted the leading edge UP a bit.
With the SNAP function all you would need to do is point at the part and press a key and the sub-assembly would SNAP to HANGER and be aligned straight forward, then press another key and the sub-assembly would SNAP to the angle of the tilt of the wing.
As some have said you can use "F" to change the GIZMO's alignment from hanger to part and back, BUT using them you are EYEBALLING the alignment.
You can use <SPACE> to zero out a parts alignment (To the hanger) WHEN HELD, but that can get screwed when PLACED, so that's why I suggested a SNAP to ALL (Basically zeroing out all axis's) after it's PLACED.