Advanced Fabrication System. This is the 'quick build' kit version with partially built foam wings.

The BVM method does involve a string as LGM Graphix suggests. The only difference is that they suggest to pull it slightly after the resin has been applied, to clean the groove. I haven't got to this stage yet but have a concern that this would move the cloth. I guess you could glass most of the wing first and leave the hinge line dry and do that separately. This would help to hold the cloth in place while you do the hinge area.

The over-riding idea is that you don't want too much (if any) resin left in the hinge line after glassing.