Duh....
It's a matter of tension.
A needle inserted into RELAXED tissue goes in and out cleaner. Less pressure to bind needle, push back out injectant or blood, and LESS PAIN.
Flexing is like trying to pull a post-hole digger out of the ground with someone sqeezing the earth to increase tension. Also, your muscle can vary in position while the needle is in (bruising and pain), or it can cramp, twitch, or any number of other motions that create bruising and pain.
Relaxed is the best way to go. Would your GF, wife, or significant other tighten up that special oriface when you go to insert? No. They loosen up so it doesn't hurt.
Sorry for the graphic image.