Tone Clusters

I just wonder how to play a tone cluster well.
Especially, I think it's impossible to press all the keys by arm at the same time because black keys are over the white ones.
I don't how Henry Cowell did it.
Could you tell me? Thank you!

It depends on how big the cluster is. If it's three notes or something right next to eachother, just make strong fingers and make sure you hit every note evenly. Don't smack the notes down though. When it's a big cluster, like 5 or more white keys, something like that, just use your arm, fist, whatever. Be creative. I don't know if you're familiar with Frederik Rzewski's "Winnsboro Cotton Mill Blues" from his Four North American Ballads, but in the beginning of that piece the preformer is required to use both his hands to play clusters over the entire lowest octave, black keys-white keys-black keys-white keys, and later on to use the upper arm for even more clusters.