well the opks detect a rise in the hormone that releases the egg. you can get a second line but if it isnt as dark or darker than the control line then it is negative. (but i would continue to DTD anyway) once you get a dark line then it means you are going to ovulate within 24 to 48 hours. so get busy as soon as you see the dark line. once you get a positive keep testing til you get a negative to pin point O day and DPOs.

also, they are notoriously unreliable and dont work for some people and follow the instructions on the pack as to when to test etc.
good luck!