It is an algorithm developed by the met office to predict areas in which the helicopter being present may trigger strikes that might not otherwise occur, therefore forecasts which predict 'natural' lightening are not relevant.
There is a page on their website which dictates whether flights can go, go subject to certain other conditions being present or absent, or not go at all. This may affect a rig or the enroute phase, cutting off access. It could be fine for the outbound, but not good for the inbound time period, therefore cancelling a flight in total.
It is very flakey and generally changes beyond recognition from the +6 hour time predicted to what it subsequently says when that time is actually here, so causes far more disruption than it should. However, if by grounding hundreds of flights over a season, the 1 to 3 strikes per year are prevented perhaps it's worth it?