Good question, to be clear, these agreements don't stop employees from leaving on their own volition but rather make it easier for the target company (the one being diligenced) to get damages from the potential acquiror if they aggressively hire away the target's employees.
In practice this often looks like an agreement to only speak with a defined, very small number of people at the target and to absolutely not initiate conversations with anyone else.
If Acme Startup Inc is quietly exploring a sale to Google, and coincidentally one of its engineers drops a resume to Google's recruiters, she can still go work for Google, but rest assured that Google will document the hell out of who-contacted-who-when.
In practice this often looks like an agreement to only speak with a defined, very small number of people at the target and to absolutely not initiate conversations with anyone else.
If Acme Startup Inc is quietly exploring a sale to Google, and coincidentally one of its engineers drops a resume to Google's recruiters, she can still go work for Google, but rest assured that Google will document the hell out of who-contacted-who-when.