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This is a medium action Dynabrade DA sander. The action refers to the size of the orbital motion, in this case 3/8 of an inch. They cost about $150 vs $50 for a cheaper model at Harbor Freight, which will choke on its own dust after a short time. I've done about five bottoms this year with this tool and it has a lifetime guarantee, well worth the money. It is imperative to oil the tool with each use when sanding gelcoat and bottom paint. Compared to using a regular orbital sander, there is no comparison in the end result- it causes almost no divoting when held flat and is easy to feather an edge. I would estimate that it cuts the time and effort to do a job vs hand sanding by a factor of six. Around chines or other sharp transitions I recommend placing protective tape as gelcoat is thin in these areas and any tool- even a hand block can sand the gel completely off down to the laminate in short order.