You should never completely straighten your bottom arm. If you do, you will never be able to swing and invert. You should put pressure in the bottom arm to create space between you and the pole.
Listen, this event is called pole vaulting but should be called pole jumping. You need to jump at take off. This with the right hand grip will make the pole bend. But, who cares if the pole bends. Guys were clearing 15 feet on steel poles because their techinque was very good. Heck, two weeks ago, I had a freshman boy clear 9-6 hold 10-6 on an 11 foot pole without a bend.
Since you posted in the beginning section, I can assume that you are new to pole vaulting. Work on your run and plant with a jump at takeoff. Put a little pressure on your bottom hand and keep your top hand completely straight about your head. As these things get better, your PR's will improve.
Remember, anybody can bend a pole but not everybody can pole (jump) vault.