Does anyone know exactly why active and NOT passive stretching is said to help prevent hamstring injury?
Maybe it is something to do with teaching the hams (antagonists in this case) to relax whilst the agonists (quads) do their work.
Whatever the reason it is worth a look if you suffer from ham problems. Some of the most well known coaches are serious about this subject and advise active stretching for the hams.
By active, do you mean ballistic, and by passive, do you mean static? I think the terms need to be defined in more detail before a useful discussion can take place.
Passive would be stretching using the body weight eg. leaning over, and active would be to contract the opposing muscle groups eg. hip flexors or quads to stretch the hams. The latter I suppose could be ballistic.