That's a good point about the boom vibrating when a load is released
suddenly. I've never seen a weight released completely as is being
proposed here, but I have seen the "headache ball" system I mentioned
in use on local demolition sites. I assume this allows the operator to
release the brake gradually, avoiding a dramatic vibration in the boom.
If I remember rightly, it was also the system used with the famous
guillotine for chopping up unwanted B52 bombers. Do a Google search for
"b52 guillotine headache ball" (or something like that) for more
details. There are a few pages out there...

Chris