While looking up stuff for a gameplay question, i found this:
Any object, everything from swords to socks to stones to scorpion corpses, can jam the floodgate this way. Once the object is removed, however, the floodgate will grind shut by itself.
I had to try this out and it's true. A floodgate will "hold" a close signal and will _instantly_ close once an obstacle is removed. That allows a small uncomplicated "blast door" to shut a passage very quickly as reaction to an incoming "activate" signal like a creature-sensing pressure plate:
R☼
║
#M#
#║#
#╚#
R - roller, driven through the gear assembly (☼) switched by the input signal. There's a minecart on top of the roller, until the roller receives power.
M - Minecart sitting in the tile of a floodgate. The floodgate has received a "close" signal and can't currently close because of the cart.
To the south, the track hook stops the cart by sending it into the wall.
Operation: if a signal is received, the gear engages, powering the roller. The cart on the roller is propelled south and collides with the cart sitting in the floodgate's tile. The roller-propelled cart stops on the tile just north of the floodgate, the cart in the gate starts moving and frees the tile, allowing the floodgate to shut. The gate closes the game step after the cart has left.
I tried this out with a suboptimal setup (too much distance from roller to gate) and got a signal-to-shut span of ten game steps. Building it like above could reduce time to five steps. Obviously, it can only be used to seal passages where you can set up rollers nearby and may leave a cart or roller assembly "outside" where it'd be exposed to enemies or collateral damage from, say, defensive magma flooding.