It is not as simple as you imply. Lincecum doesn’t just extend his glove arm to the side. He counter-rotates his whole body as he goes into knee lift and his glove arm aligns itself accordingly. Does that help him stay closed? Possibly. But, for Lincecum, it is probably more related to achieving maximum hip and shoulder separation.
If you’re having trouble staying closed then you need to identify the cause before you can determine a fix.