Spark....not trying to be a pain in the butt, but it does matter....I'm not clever enough to demonatrate....mayhaps someone can?

Hi Rich
It really  does not  cut a  warp in the  wood. I have done this on every plane I have built for the past 30 years, NO Warps.
example  Flaps, mine  are  3/8 inch  tapered to 1/8 inch
I take a 3/8 inch rod and put  it at the lead edge or hinge line, this stops any cutting of the  flap at that end.
the trail edge gets a 1/4 inch rod, I sand this side with rough paper and it will cut an even taper of 1/8 inch on one side of the  flap.
I then turn the flap over, and put a 1/8 inch  rod at the trailing edge which is now 1/4 inch thick, I sand this side and it cuts a 1/8 inch taper into the flap.
This is now a flap that has a 1/8 inch taper cut evenly in both  sides, they come out straight with  ZERO  warps cut into them.

100s of people use this rod trick  and  have  no problems,  The only ones I have seen have problems are not following directions.

Now if your using a  curved  trailing edge, you can get in trouble, you have to bend the  trail edge rod to the same curve as the  flap.
You can also get in to problems if you push down hard on the  sanding block...example, your sanding on your card table, when you push down hard it  warps the table, since the flap is laying on the table it will warp  too.
So you just need to be careful.

Randy

You will need to over bend it some. I would use steam instead of water.

Sand with your right hand and hold the wood with your left.
You can also use key stock of the appropriate size ,the square shape is a little easier to hold steady.

You can also make a jig to hold the rods steady ,use a piece of plastic laminate covered plywood or a piece of shelf stock, 4 to 4 1/2 inches wide by 24 to 28 inches long.
Drill holes that will fit a small finish nail at 1/4inch on center at each end.
Put 2 nails at each end ,adjusting them to fit the taper of the stab or flap, intall the rods ,slide the part in and you don't have to fight stabilizing the part while sanding.

The biggest thing to remember while doing this is to let the sand paper work, dint bear down or the part will WARP or cup.