Utricularia

The trapping mechanism of Utricularia is purely mechanical; no reaction from the plant (irritability) is required in the presence of prey, in contrast with the triggered mechanisms employed by Venus Flytraps (Dionaea), waterwheels (Aldrovanda), and many sundews (Drosera). The only active mechanism involved is the constant pumping out of water through the bladder walls by active transport
As water is pumped out, the bladder's walls are sucked inwards by the partial vacuum created, and any dissolved material inside the bladder will become more concentrated. The sides of the bladder bend inwards, storing potential energy like a spring. Eventually, no more water can be extracted, and the bladder trap is 'fully set' (technically, osmotic pressure rather than physical pressure is the limiting factor).