"Silva" (one portagee, eh?!)...just kidding. What you saw was a stan pleskunas "rocker stick". Fiberglass Hawaii USED to have them, but they do not carry them any longer (you can give 'em a try, though). Be prepared to shell, however,... they were 450 bucks when offered as new. T.
The track that the feet fall into is slotted (extruded plastic)so the length of the track gets longer the more rocker there is. This is what keeps it from bunching up. I use scraps of balsa from top or bottom rocker to lay along the bottom or deck of the board and then trace the curve. It takes several trimmings until it is settled nicely into the curve. Once you have one of these, it is only a matter of expanding this curve on another piece to create a longer or shorter piece to make a new set of curves
A rocker stick is a good tool If you need one. The main advantage of a rocker stick is if it is robust enough and is accurate you can use it with a template guide and a router to create plywood patterns/templates for use when shaping. This is the focus of my design of this tool. It can be adjusted and used as a guide when shaping but it is cumbersome and attempting to re-set it with marks on the rods is nearly impossible to do accuratly or repeatably. That is why it needs to be used to make wood patterns that are light and easy to use while shapeing. I use a fibeglass batten and alum rods that go through a plastic dowel the dowel is drilled and tapped on the axis for a set screw and also at 90 degrees to the axis for the rod to pass through. The back bone of the stick needs to be strong enough to resist the tension of the batten and drilled on exactly the same centers as the clevises that hold the batten end of the rods. The rods must piviot at the batten and they must also pivot where they go through the back bone. If the piovting elements work smoothly the tension in the batten can be used to fair the lines you are looking for by loosening and and tighting the set screws individually once you get the stick adjusted. The batten needs to be strong enough so it doesn't sag between the rods with pressure from the router while cutting patterns. This means that the batten needs to be fairly thick and then tapered at the nose end. If done correctly the increased tension in the batten where it goes over the nose rocker takes care of the tendancy to sag where the batten is thinned out. I use mine constantly for a lot of things other than surfboard stuff. I find it is most useful for making fair concave lines. All of our tools work great on convex or straight lines but the concave stuff is a hassel. I quit making rocker sticks because it isn't cost effective to make them one at a time. The cost of the required machineing and materials to do it the way I think is best, requires me to build about 25 of them at a time. Even then to make it worth my while the price is beyond what most guys want to spend. With that in mind there are a lot of boards that have been shaped without them and there are other methods to achive the desired result more inexpesivally. I think the explainations here are all great and if you are looking for accuracy a stick can certainly help. However if you consider how much blanks move after they have been shaped and glassed the results are only approaching repeatability. I wouldn't want to hazard to guess the tolerance of this repeatablity with a number, but I bet you would be surprised by how big that number is. The way to test it would be to scan a shaped blank then glass it, consider the required offsets for hot coats, glass etc. then scan it again and measure the differences. I would like to see a study that did this for ten identical CNC boards. That might take all the magic out of it though. The real question is what is an acceptable tolerance in a rocker to get the same results when in the water?
This would be very easy to modify to copy rockers off of a proven board.