ACR MLG ALT DEV EXCURSION FROM CLRNC ALT IN RESPONSE TO TCASII RA.

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WE WERE S OF STAPLETON ON AN ASSIGNED HDG 080 DEG AT AN ASSIGNED ALT OF 11000 FT. DENVER APCH ISSUED US VFR TFC 12 O'CLOCK 11500 FT. I BELIEVE DENVER SAID THE RANGE WAS 6 MI. TCASII DISPLAYED A TARGET IN THAT POS AT 11500 AND CLOSING. A MOMENT LATER DISPLAYED (TCASII) ALT FOR THE TARGET INDICATED 11400 FT; THEN 11300 FT. WITH THE 11300 FT INDICATION APPEARED A DOWN ARROW. THE DOWN ARROW MEANT TO ME THE TARGET WAS NOW DSNDING AT LEAST 500 FPM. I ESTIMATE RANGE THEN AT ABOUT 2-3 NM. WITH THE DOWN ARROW TCASII ISSUED A DSND RA. THE COPLT; RESPONDING TO THE TCASII RA AND MY COMMANDS; INITIATED A DSCNT. I NOTIFIED DENVER THAT THE TFC WAS LEAVING HIS ALT AND DSNDING INTO US. TCASII THEN ISSUED A 'MONITOR VERT SPD' RA. TCASII NOW SHOWED THE TFC CLBING AGAIN. THE LOWEST ALT I SAW FOR THE TFC WAS 11100 FT. DENVER REPLIED 'THE TFC IS VFR AT 11500 FT'. I INFORMED DENVER THAT TCASII SHOWED THE TFC HAD LEFT HIS ALT AND DSNDED INTO US. I TOLD DENVER WE HAD LEFT 11000 FT IN RESPONSE AND ASKED IF HE WANTED US TO RETURN TO 11000. DENVER REPLIED; 'NO STAY AT 10000 FT NOW'. THE ENTIRE EVENT TOOK PLACE VERY QUICKLY. FROM MY PERSPECTIVE THE VFR TFC MADE A VERY RAPID ALT EXCURSION FROM 11500 TO AT LEAST 11100 FT AND BACK. THIS WAS A TARGET VERIFIED BY DENVER RADAR. IN MY MIND IT WAS A VERY REAL TARGET. A TARGET THAT TCASII SAID WAS CLOSING RAPIDLY AND DSNDING INTO MY ALT. I HAD NO TIME TO SECOND GUESS. FOR ME TO HAVE IGNORED A TCASII RA ON A TARGET THAT HAD BEEN VERIFIED BY DENVER RADAR WOULD HAVE BEEN GROSSLY IRRESPONSIBLE AND STUPID. IN THIS INSTANCE I BELIEVE TCASII PREVENTED A MIDAIR COLLISION. THAT I AM NOW THE SUBJECT OF A PLTDEV RPT BY DENVER ATC DISTRESSES ME.