The term implantation dip is often used to refer to a luteal phase dip that occurs around the time of expected implantation (7-10 days past ovulation). While this pattern does not always result in pregnancy, the term is often used because of the timing of the dip and because this pattern appears with greater frequency on pregnancy charts than non-pregnancy charts. There are a couple of factors that may help to explain why this pattern appears with greater frequency on pregnancy charts.
The corpus luteum (which produces the heat inducing hormone, progesterone) normally peaks in its production of progesterone and then begins to recede around the middle of the luteal phase. In conception cycles, it is "rescued" when the embryo implants and then continues to produce progesterone until the placenta can take over hormone production.
Estrogen, in opposition to progesterone, has a lowering effect on temperatures. A secondary estrogen surge in the middle of the luteal phase may cause a temperature dip at this time. Indeed this may explain why this pattern also occurs on non-pregnant charts. Mid-luteal phase estrogen levels, however, have been found to be higher in conception cycles than non-conception cycles and this may also contribute to the greater frequency with which we see this pattern on pregnancy charts.
When you see a mid-luteal phase dip on your chart, however, it does not necessarily mean that you are pregnant. Likewise, you do not have to see this pattern to be pregnant. As long as you have intercourse in your fertile time, you have a chance to be pregnant with any ovulation pattern.
See also a statistical study on this topic.