The Instant Eviction is a twist introduced in BB1, returning for multiple seasons throughout the series.
Details
The Instant Eviction works the exact same as a normal week would, except no Power of Veto competition takes place. This means that after an HOH names their initial nominees, an eviction will immediately begin.
In BB10 and BB13, a different version of the Instant Eviction occured as a result of an opening twist, where lower scorers in competitions would be nominated without the chance to save themselves.
In BB12, the Instant Eviction occured as part of a double eviction.
Effects on the Game
This twist dramatically changes how a week plays out. With no POV to compete in, plans on the trajectory have to change. A notable example of this is in BB4, when Lorde won the HOH and wanted to backdoor Quaking, but was not able to after it was announced that an instant eviction was happening. As a result of the instant eviction's occurence, she had to change plans and shift targets to DoS.
Player Reception
The instant eviction twist is more of a mixed bag of reception from players. Though it was used frequently in earlier games of the series, it's often called into question how fair it is, not giving the initial nominees a chance to remove themselves from the block. An especially odd inclusion was BB4's Instant Eviction, given that it was the first game to completely remove any kind of comeback mechanic.