Keilgezinkt could also be dovetailed, which I think is more likely here. A fingerjoint that is not glued would not prevent the joints from coming apart in either direction, whereas a dovetail joint that is not glued will not come apart in one direction.
Kraftschlüssig is a friction fit or pressure fit. In this context I think it means held together by the friction of the joints as opposed to being glued, which means that the formwork can be knocked apart with a hammer.

http://www.proz.com/kudoz/1144028