What is it?
You've channeled fire, ice and electricity out of your flesh... why not wind? Pinpoint a spot on the ground and this plasmid will produce a swirling, permanent tornado there. Anyone who passes through it will be rocketed helplessly towards the ceiling. Comedy gold.
When should I use it?
As the name suggests, Cyclone Trap is designed for ambushes. You're not shooting gusts of wind straight at enemies, but placing dangerous hot spots on the map like you would with proximity mines or trip wires. Therefore, the best time to employ this plasmid is when you know - or think you know - exactly where a Splicer is going to go. Door frames, narrow hallways, catwalks and any other confined space is perfect.
Cyclone Trap can be used effectively in the heat of battle... you just need to know what you're doing and practice, practice, practice. The trick is to aim not at the enemies themselves, but at the patch of ground directly in front of their feet. The tornado takes half a second to appear, so you'll need to lead your prey like you would with a sniper rifle. NEVER use Cyclone Trap in close quarters combat.
Here's something to try if you're feeling adventurous. Throw a proximity mine onto the ceiling (or move it there with Telekinesis) and then set up a Cyclone Trap immediately below it. Splicer goes up... Splicer comes down, nicely toasted.
Does it work against a Big Daddy?
If used correctly, yes. The Big Daddies are far too massive to be troubled by a little gust of wind, so don't expect to send one flying through the air. For some reason, though, they can't move through the tornadoes.
By creating multiple Cyclones in a row, you can form a wall that effectively and permanently blocks the Big Daddy's progress. He'll still be able to shoot and lob grenades at you, but the threat of charging is removed. With enough space and maneuvering, you can even build an elemental prison around the Big Daddy, trapping him in one place forever.