I find it helpful to have some idea as to why things work instead of a pile of 'recipes',. So, IMHO:
Maple maniacs sometimes just bury the graft union to root the cultivar. It seems that there is enough of an auxin pile-up to always be generating root initials at the union interface zo that all one must do to generate adventitious roots is just keep the union damp. Maybe it happens with spruce as well, but you'll have to try for the world to know (as I've not found any internet declarations about it one way or the other).
The basic thing that must happen to generate root initials is to have enough auxin pile up to up-regulate a cambium cell. Once this happens, it begins to produce auxin that cycles upward in the cork cambium and back down, just as happens at a root tip. It is thanks to Polar Auxin Transport that auxin can 'pile up' to make roots grow. Girdling a stem by removing a ring of bark or by the application of a tourniquet interrupts the PAT and causes auxin to pile-up. Then it is just a question of whether the cambium cells can up-regulate and turn into root cells.
Auxin also serves as a messenger indicating 'life above'. When the signal disappears (which happens with pruning, for instance), it triggers the death of cambium cells and the release of hormones that cause CODIT reactions to occur. The cambium immediately below a girdle or tourniquet is vulnerable to this as the only way auxin exists there is that is 'pumped up' in the phloem flow from below and then off-loaded into the PAT stream. Cutting flaps instead of girdling a stem is clearly a means of continuing the flow of auxin to the cambium below so that the layer doesn't fail due to CODIT reactions sealing off the xylem flow (which carries water and essential minerals from the roots). However, this also saps the accumulation of auxin above which tends to prevent rooting. I've played with doing this on several species and am convinced it rarely, if ever, leads to the production of adventitious roots. Again, it is something you might try, but I predict certain failure = prove me wrong ( = I like learning about things, but am not going to spend any more of my time on such 'non-sense'
)..
There is always some auxin getting to the roots, but it is minimal during dormancy and when extending new growth. Buds and leaves produce auxin, but when new growth is extending, it is largely consumed within that new growth (making it extend). Once the new growth has hardened, it is highly productive in terms of auxin and carbohydrate supplied to the rest of the tree. Hence, root growth is strongest after new growth has hardened. Hence, root growth is strongest in spruce not long after the summer solstice, pretty much as
@TomB said.
Lastly, the auxin flow tends to be higher on the underside of a stem than on top, especially when it stem curves. If you bend a stem (including spruce!) downward, it tends to release a bud that is at the top (i.e., the highest point above the ground) of the bend. Conversely, a horizontal stem tends to curve upward because more auxin flows on the ground side of the stem and makes new cells elongate more than those on the top of the stem. It also can lead to the production of adventitious roots on the underside of a stem that is kept damp (which is usually because it is in contact with the ground). Anyway, my suggestion is to try bending a stem into a 'U' shape and cut part way through the stem on the bottom side at the bottom of the 'U'. Put a toothpick of the like in the cut to keep it open and pack it up as you would any air layer attempt.
