I haven't used them but I thought the digital OPKs just told you in words whether you're having your LH surge like the CBD HPTs say pregnant or not pregnant. The non-digital CB OPKs have one control line and one line showing the LH surge - you're having your surge when the LH line is darker than the control line. If you've no LH surge you either don't get a second line or it is not as strong as the control line.