Why does straight arm cause biceps hypertrophy?

No, when a muscle is fully extended - bicep in a straight arm or triceps in fully bent arms - it is weaker, so it has to work much harder. This is the reason some people can hold a planche with slightly bent elbows but can't straighten them (ps this is never a way to train the planche)

Alan Tseng said:

Since straight arm means elbow is extended, doesn't that mean it should cause triceps hypertrophy instead and bent arm cause more biceps hypertrophy?

 

But why is that gymnasts gets so much biceps hypertrophy from straight arm strength exercises?  Don't really understand that part

I would have to say that long term straight arm work definitely causes growth in bicep mass (my wife will testify happily to this!), however, I personally did see some odd things when looking at what I could lift after doing primarily straight arm strength work for a long time.  

For instance, for me (and I was never great or that strong) I could easily curl a 60 pound dumb bell to about 50% of the way from a dead stop with my arm completely straight, but could only get a 45 all the way.  That should all be neurological though (I never tested it personally, I didn't care enough) and a few weeks of full range of motion weight work should have spread that strength through the entire range of motion, once the neurological side got a bit more trained in those sections of motion that I neglected in my general workouts.  

The speed with which neurological adaptations change (just a few weeks to my understanding;they comprise that first few weeks of massive gains people make before they plateau against muscle and connective tissue weakness) made it seem pointless to me to worry much about it.  But depending on your sport, it may be something to think about.