This is you essentially performing a valsalva maneuver. Your larynx closes (trying to say a vowel) and your stomach tightens. This compresses your air and creates that tension. I don't think the larnyx is something that can always be controlled as a stutterer. We can activate it sometimes but not at other times. Relaxing may help but I haven't had much success with it. I think we can control our mouths and tongues but just not the larnyx sometimes. I think (if this is how it is for you) you can just accept that that is the cause and know that that aspect can't be controlled. Doing this can help reduce anxiety which is a major cause of stuttering.