Refer to the question in the title, as I would like to know the answer.
I don't think it's necessary to "lock up" the hands to play a buzz. I play my buzz rolls with my hands as relaxed as possible. Granted, a certain amount of pressure at the fulcrum can help with buzzes depending on volume and the texture desired, but I'm not so sure giving the impression that you need to "lock up" your hands to get a good buzz stroke is good advice. If you're too tight, your buzzes will remain short and scratchy sounding - which can be useful - because your tension is actually impeding the stick from moving/buzzing. So for long velvety rolls, it's useful to back off on the tension and let the sticks breathe a little, in my experience.What Jeff said.
Remember, that when you do a closed roll you naturally lock up - or close - your hands to get the buzzes to blend into one another, while the hands are loose and relaxed for doing the open rolls.
Sometimes though I hear even the greats do pulsey buzz-rolls. It might be good in some band situations when a 'clean' closed roll could lose the pulse of the song. At least that's how I have interpreted it. It could be also a matter of taste or they might just not be able to do it more cleanly in the given situation and energy, I don't know. But I have heard it many times. What do you think?But, this makes for "pulse-y" rolls in which you can hear every initial attack. I've found I get better results by striking that initial stroke with less impact and using relaxation to lengthen out the buzz.