The judges rank the couples 1- however many in the comp that week by awarding their marks. You have one couple with perhaps 21 points and the last couple with 17 points. If there are 5 couples then the couple with the highest score will get 5 points descending to the couple with the lowest score getting one point.
The public vote for their favourites and the couple with the most votes get 5 points descending to the couple with the least votes who get 1 point.
The two points scores are then totalled to give an overall score.
For Example..... A B C D and E are the competitors
The judges award marks thus.... C = 25.5
A = 21.5
E = 18
D = 17.5
B = 17.
C gets 5 points
A gets 4 points
E gets 3 points
D gets 2 points
B gets 1 point.
The phone votes show that.... B got 32,500 votes
A got 28,000 votes
C got 22,800 votes
E got 19,000 votes
D got 2,000 votes
B gets 5 points
A gets 4 points
C gets 3 points
E gets 2 points
D gets 1 point
Add the two lots of points together and Bob's your Uncle..
C has 8 points
A has 8 points
E has 5 points
D has 3 points
B has 6 points.
With the least points D and E are in the bottom 2 and face the judges decision.
Sorry if it's longwinded but I hope it helps...