Pressure can be 'transmitted' north, south, east, west, above, and below. If you get water down to its final level using a long straight staircase, then the water will be pressurised between the source level and the bottom of the staircase at the very least. After you force it to move diagonally using baffles or a kink in the tunnel*, it is no longer pressurised. It'll run down levels if you give it the space to, but it'll be much slower than water that has a source pushing it around. If you use a pump the water will be repressurised to the level the pump sits on and will be capable of rapid flooding, but only to the level of the pump (this is less than comforting if the pump is above all the important areas of the fortress).
*These are pretty much the same thing to the game, but may be considered as conceptually different.
Here's an example (top down view):
##################
%%<<<<<<...!<<<<.#
########...#######
#...#
#####
What we have here is a pump leading to a 3x3 reservoir, with a smaller reservoir off to the right. The ! is the location of the fortification carved to link the two.
When the pump is off, it is safe to carve the fortification. The water will spread out naturally, forming a gentle slope, and areas of 6/7 water will start to appear left of the 3x3 reservoir as the water level tries to equalise across the whole tunnel.
When the pump is on, it is not safe to carve the fortification. The water will travel through the fortification and beyond much faster than a dwarf can move, and there'll be two or so tiles between dry floor and 7/7 water, meaning that the water that does come through is almost instantly at drowning depth.