Some will allow the tips to passively bend back for free strokes as well, but it's more common for rest strokes. It's really the flexion of the PIP (middle joint) while the DIP(tip joint) is allowed to be loose that makes the DIP bend backwards. I'm on the fence about it for free strokes and tend to not do it. I see free stoke tip joints collapsing most with the middle finger, likely because it's the longest finger and it makes it easier to get the finger to push through the string that way. If you use a free stroke technique where it's only PIP/DIP that are pushing through the string and MCP(large knuckle) actually extends at the moment the string is sounded, that seems to help to keep the tip joint from collapsing.
An eyewitness will often only see what he already believes to be true.