Double Exposure on Just the Ace up

I was playing at some of the South Lake Tahoe Casinos recently and noticed an interesting rule at the Lakeside Inn. SD, double on 10 and 11 only, 3:2 BJ, Hit Soft 17, no double after split(that's pretty typical for some of the places up in Reno and Tahoe). Here's the weird part. When the dealer had an Ace up, he/she asked for insurance, and after that decision was made, he/she flipped over the hole card without peeking, and then the players could play out their hands from that point (so long as the dealer did not have a BJ). Double Exposure on just the Ace, go figure. Does anybody have any idea how much this adds to the players advantage? I didn't know, but was just curious and wanted to find out. Thanks.