The original question was "fixed IP"...
In a KISS system, the IP is fixed to deliver a constant flow eg ~.8lts/min and this can be achieved by blanking off the regulator from external/ambient pressure thus ensuring that the first stage will deliver a fixed line pressure (IP) regardless of depth (up to the IP pressure).. that is, if the IP is fixed to = 10bar, then the IP will stay constant at 10Bar and you will have flow though a valve to a depth of 90meters. Once you exceed 90 meters, the flow will stop as the pressure external will be greater than the IP.
In a non fixed IP, ie a normal regulator first stage, the IP will be fixed ABOVE ambient pressure. That is, if the regulator is set a IP=10bar, then the IP will rise to 12 bar at 20meters. (This will obviously affect the flow rate of gas through a valve hence the IP is fixed in KISS valve systems...)
I hope this helps, otherwise I will link you to other articles to explain more fully.... good luck!
Cheers, JDZ
Edit: Sorry guys, I think I missed the original point of the thread...
The reversal of flow, apart from depressing the purge, will depend on whether the valve is upstream or downsteam...