E.g., there's a 5 person team responsible for a critical component. 4 of them leave in the space of 6 months. So management says to the last one "we'll give you $200k at the end of 12 months." That person then leaves in exactly 12 months and a day - which is fine, because they trained the new team on the critical component.
That is why I feel this bonus is not a good idea, $50k/year is not enough comp bump to hold someone in the Bay Area
He was working at another well-known tech company that is suffering from a brain drain and he was getting a lot of exposure (including in the press). They gave him a seven figure retention bonus in RSUs, and he used that to get around the same from a competitor where he felt he would have better long-term prospects. Overall his yearly expected comp went up by about 50%, and should be around $750k-$900k at this point.
This strategy could work if the bonus was big enough to outbid most of the competition.