I was discussing techniques with a tailor by email and was having trouble describing something so I thought “blog post”.
We typically find two types of tailored sleeve in men’s suiting- English tailors often cut what is known as a 50-50 sleeve, whose under sleeve is roughly the same width as the top sleeve. More common today is a sleeve with what is known as a “false forearm” because the forearm seam is offset from the from of the sleeve by around an inch in order to conceal it. The offsetting of this seam can cause a kink near the elbow if the sleeve is not shaped properly. The under sleeve is cut roughly 1/4″ longer than the top sleeve along the forearm seam; some tailors work this fullness in and shrink it out when pressing the forearm seam. I have a different take on this.
Notice the concave shape of the forearm seam when the sleeve is flat.
Now when I turn the seam back to replicate the offset of the seam in the finished sleeve, notice that the front of the sleeve is straight and the seam is now convex, rather than concave- this is what can cause the break in the sleeve.
To counter this, instead of shrinking out the fullness on the under sleeve, I stretch the top sleeve using a steam iron or by moistening the cloth to within 1 1/2″ of the cut edge, pulling on the cloth as I hold the sleeve as shown (this can also be done after the seam is sewn, when opening the seam). 
Notice how the edge of the sleeve ripples because of the stretching. The top sleeve seam is now 1/4″ longer than it was, and even with the under sleeve seam.
And now when I fold back the front of the sleeve, the fold is now nicely curved and the seam is now the proper, concave shape.
apparently some pictures (3) didn't showed up
Really? Which ones? They all show up on both my Mac and PC.
J
Mr. Diduch: a long-overdue thank you. I am in the home stretch of making a tuxedo for my son and would never have even attempted it without the information from your blog. I have read it through at least twice and can't thank you enough for sharing your knowledge with those of us who are newcomers to men's tailoring.
Hello Jeff. Thank you for sharing this information on sleeves. The sleeves on your finished jackets are always shaped so beautifully; a nice lean natural curved shape. I steam and sew my sleeves using the method you just shared with us, but my finished sleeves on my jackets never come out as nice as your sleeves.:-) Should I be doing something differently?
Hi Jaff thanks for information
Jeffery you really know your stuff. I learn much from you. Thx. Frank.
The iron work (magic) of tailoring is really fascinating and I appreciate information you've shared on your blog.
How do different fabrics effect shaping? Would one perform similar stretching/shrinking with linen or cotton? Would it be possible to alter the pattern piece of the sleeve so it has a more concave curve to start?