There are 2 types of OPK, short for Ovulation Prediction Kit, on the market: urine test based and saliva test based. The urine test based OPK is a simple test that detects the surge of Luteinizing Hormone (LH) in your urine.
There are 4 stages in your cycle: manstruation, follicular phase, ovulation and luteal phase. LH remains low and steady throughout the manstruation, follicular phase and luteal phase. However during ovulation, LH spikes up dramatically prior to the release of a mature egg.
This surge is what OPK is designed to detect. Testing positive with an OPK urine test strip means your body is about to release an egg within the next 24-48 hours. So your best chance to get pregnant is to catch the egg right when it is released, which is 24-48 hours after the detection of LH surge.
Once the egg is released, it has a life span of 12-24 hours. So don't just stop there after 48 hours. Keep up the BDing for another 24 hours. In fact if you have the stamina, BD twice a day, morning and night, 4 days in a row straight from the moment the surge is detected. After all there is never too much sex for men, or is there?