From my under standing, the flood gate control determines when the low speed compression damping is over ridden.
Example. If you run no low speed compression (blue dial on left hand leg) the Floodgate has no effect at all, nor do you obtain any lock out).
With some compression damping dialled in, the flood gate control can determine how much force is required to make the forks active, so you can effective stiffen the forks/slow them down until sufficient force activates them.
I run the required compression with a little flood gate (5-6 clicks out of the 20) so I have a very active forks over the small bumps, with enough compression /4-6) clicks so stop them diving. I run the PSi at 70 in both positive and negative chambers and I weigh 160lbs.