Currently, I try to improve the power in my smash beside pronation and body rotation. I feel that when I perform a power smash when I hold my breath and flex my abs there is more power generated into my shot [from the sound it made and the opponent reaction]. I 've read some where that when we hold our breath or even yell when we smash our core body becomes more solid which could improve the quality of the smash. Is that true?
It is a general principle in any sport to forcefully breath out and contract the abs during a forceful action. This is most commonly taught in martial arts, but it holds true in any sport. In badminton it can help your smash, clear, backhand clear, the explosive initiation of movement from your base position. With any movement where your body is being extraordinarily forceful, the flexing of the core and breathing out helps make the movement more powerful. The shouting is really just an expression of effort while forcefully breathing out.
SnowWhite has summed it up, but here's Anna Rice advocating the same thing: This is also given as a tip in a Badminton Famly video, although they don't really explain why it's helpful:
