Having sewn for boy's for quite a few years I was thrilled to learn this technique for Nancy Zieman.
You want to control the fullness at the top of the sleeve. So, do not let the fabric feed through, keep your finger at the back as you sew. If to much fabric is piling up, you can let it go, and continue again with your finger in the back of the pressure foot. This also works on girls blouses where you want a smooth top to the sleeve.
With this done, take the sleeve to the ironing board and steam it very well. Do not! touch the iron the the sleeve, just give it a good shot of steam to shrink out the fullness.