I thought it was a guarantee he would not be released without ICE dealing with the situation...whatever it is. I could be mistaken.
I looked it up to be certain and it appears correct he would go to them instead of being released on bond because of the serious nature of his crime. This explains it much better.:
The hold notifies the facility to not release the person, but instead to transfer him or her to federal custody at the end of the jail term. An immigration hold may be placed after either a random check by ICE officers (who sometimes visit jails and interview inmates_ or after being notified by the law enforcement agency detaining the person. When DHS places a hold, state law enforcement agencies are obligated to comply.
~snip~
Mandatory Detention for Certain Crimes
For certain crimes, detention in a federal institution is mandatory. You will not be released before the completion of removal proceedings or the carrying out of a deportation order if your record shows:
aggravated felony charges
http://www.alllaw.com/articles/nolo/us-immigration/hold-process-jail.html