This is the rule with the most profound impact on the Playing Strategy. It amounts to the ability to Double Down on the 2nd card that you get after a split, for example with the 4-4 should you get a 5,6,7 you would double with the dealer's up card; (4,5,6). being the appropriate action. Hence you split much more often, when das is offered.
Hand
2
3
4
5
6
7
8
9
X
A
2-2
S*
S*
S
S
S
H
H
H
H
H
3-3
S*
S
S
S
S
S
H
H
H
H
4-4
H
H
S*
S
S
H
H
H
H
H
5-5
D
D
D
D
D
D
D
D
H
H
6-6
S*
S
S
S
S
H*
H
H
H
H
7-7
S
S
S
S
S
H*
H
H
H*
H
8-8
S
S
S
S
S
S
S
S
S
S
9-9
S
S
S
S
S
N
S
S
N
S*
X-X
N
N
N
N
N
N
N
N
N
N
A-A
S
S
S
S
S
S
S
S
S
S*
Note the variation explanations provided with the MouseOver event, by placing the mouse pointer over a "S*". Here we can see that the most important rule variation is that of the ability to Double after you split, (i.e.,das). The really important splits for this effect are the small pairs. The 8-8 and 9-9 are split in a defensive mode, hopes are that the 16 and 18 can turn into a better pair of 8's and 9's. Needless to say a 2,3 or Ace giving a chance to Double after we split.
Also note the differences in 1,2 and 6 Deck for the 6-6 vs the 2 and the 7. And the changes associated with the Splitting of 9-9 vs the Ace. And interestingly the A-A vs the A. For a complete dynamic visualization of the basic changes with respects to the Rules click here.