hz (hertz) is cycles per second, which in a pulsing frequency context would be 60 times smaller than ppm (pulses per minute, or cycles per minute.)
My machine (an older Mosfet-based Everlast "Super200P") goes up to 300 hz pulsing frequency. the 300Hz pulsing is helpful for constricting, focusing or "stiffening" the arc, and making it penetrate more deeply and not having the heat spread out as wide.
But a skilled welder can accomplish similar effect without using pulsing at all, by simply holding a tighter arc length (holding the tungsten electrode closer to the work.) So, its best to learn to weld without pulsing, to develop manual heat control before experimenting with pulsing.
Super high frequency pulsing (as high as 10 kilohertz or more) has been found to increase arc pressure. This article has some other interesting facts about high frequency pulsing. http://www.thefabricator.com/article...tube-producers
I can tell you that in actual use, high frequency pulsing (300Hz) has an effect in DC welding that is helpful to keep the arc more directional and focused. It can be helpful to prevent the arc from melting a nearby edge. (See attached pictures, showing 1/8" 1018 steel flatbar I welded with 300Hz pulsing, close to an edge without melting the edge.)
My machine, which lacks adjustable A/C frequency (its pre-set at 60Hz), also benefits on low amp aluminum welding in some joints from the 300Hz high frequency pulsing as well. I feel the 300Hz pulsing setting helps focus the AC arc and perhaps partially makes up for the machine not having adjustable A/C frequency.
The adjustment on my pulsing frequency knob is kind of funny in that its not very linear, so its hard to set anything other than the minimum or maximum pulsing frequency. Most other knob settings seem to be just a small bit above minimum. However, I basically just use it maxxed out (300Hz) or at minimum (of perhaps 1.5 pulses per second) and I don't feel not being able to vary in between those two settings is holding back my welding abilities very much. I'd say having both a relatively high frequency pulse setting, and a relatively low frequency setting is useful, however. They are useful in different scenarios.
Low frequency pulsing can be helpful for limiting heat input, for example, if you are welding thin sheetmetal and want to minimize distortion. Whereas I usually use high frequency pulsing more when I want to make the arc more directional, contricted, and "focused".
Richard 
