This is my understanding.... On chesil the tide floods from right to left and ebbs from left to right. However due to Portland the tide will continue in the same direction for some hours after high and low. So essentially the tide could still be flooding (right to left movement) but going out and the opposite on the ebb. The tide will come to a complete stop mid tide cycle before changing direction. The timing changes depending on the tide size, so on a spring the tide stops a couple of hours before high water, starts to flood and continues for up to 4 hrs into the ebb. Neaps are more predictable, you generally only get tide movement over low and high, with hours of dead, slack water in between. This is why chesil is generally poor on neaps, unless you can grab a few over high or low. The further east you go the stranger the tide gets, with almost no tide on the spring ebb and all flood. Then more tide on neaps, this is due to the water being deflected by portland and missing out a large part of the beach. The two highs are the top of the tide and then when the tide stops moving from right to left. The first low is the lowest tide point, then the second when the tide stops it's left to right movement.
