"Maintain Vertical Speed, Maintain"

We departed Punta Gorda, FL (KPGD) enroute to Houston Hobby (KHOU) and were cleared to climb to 11,000 feet. The weather was typical for southern Florida in the spring time—2000 Broken and 10 miles.

A couple of minutes after takeoff, while climbing above the broken layer, we received a TA almost immediately followed with an RA. This was not your typical Climb or Descend corrective RA. It was one I’d never heard before: "Maintain Vertical Speed, Maintain."

The RA was very explicit, and I followed the verbal instructions by maintaining my current rate of climb and keeping the VSI in the green arc. A few seconds later, our TCAS announced, "Clear of Conflict." Shortly after that, the departure controller advised us of VFR traffic at one o'clock passing from right to left that should be no factor. Better late than never.

At the end of the day, I did a little research regarding the RA that we had received. I learned that Maintain Vertical Speed (or Crossing Maintain Vertical Speed, when intruder's altitude will be crossed) are only issued when your aircraft is already climbing or descending, in the correct vertical sense, at more than 1500 feet/min.