The yellow plastic thing on the drinking straw is a whistle and not used as a clip or anything..
Yes you do need to bite down slightly to open the mouth piece to be able to drink.. or it would leak..
btw.. The blue mouth piece on the end of the drinking straw has a slot that is often sealed when new..
Here's a link to a sup review..
www.paddlemonster.com/2014/09/02/gear-review-camelbak-molokai-hydration-pack/