Somewhere on the internet I have read comments about cracked Plugins being "time bombed". What is this?

 

It means the software reverted to Demo mode, even though it seemed to be properly cracked.

 

At first, as the name say "time bombs" would be simply triggered by time, the time you used the plugin, or a specific date.
Today this protection technique have eveloved, mixing time and use of specific parameters.
Example: you've be been using the plugin for one week + you've twisted the filter knob 5 times + you saved 2 presets = plugin locked.
This can be programmed as complicated as the developer wants.

All these are deeply hidden and need a lot of test from crackers to discover each time bomb. That's why it is used.