Police Radar Gun - MPH K55

I was on my out of this sale when I saw this sitting there.  The off-duty female cop shouted from the garage, "It's a radar gun, three bucks".  "How can I resist?" was my reply.  She showed me how to work it, but I forgot what she said and I couldn't figure it out.

POLICE RADAR GUN UNIT FROM PATROL CAR MPH K55  ends May 12

Update:  Sold for $43.86