Jack overcomes Ralph's control through the use of fear.  The boys are extremely paranoid about the beast, especially after Samneric see the parachutist on the mountain and mistake it for a horrible creature.  Jack uses his prowess as a hunter to bring the other boys over to his side; he convinces them that as a hunter, he will be able to stalk and kill the beast.  Since Ralph has little to no skill at hunting, Jack draws attention to himself as being both a better protector and provider than Ralph. 

In chapter eight, Jack leaves the tribe in a sulky fit but quickly rallies the older boys who enjoy hunting to his side; Jack seizes power through fear and manipulation.