I bought one of these on Ebay to see how they worked. Very impressive. Easy to keep on line. Still have to get a better feel for distance but it was worth the $69.
Inazone Armbar Armlock Design Putter - Winn Black 15" Grip | eBay
Inazone Armbar Armlock Design Putter - Winn Black 15" Grip | eBay