I described this feature briefly in Getting More Out of IDeA in Aquarium 2010:

> "Generate Alternatives" is used in connection with "Alt tree." When it is
> selected, each position that is sent to IDeA will be used to find an alternative
> move instead of analyzing the position normally. The move in the game and
> the moves that are found in the "Alt tree" will be excluded in the search for
> the new alternative. One method of using this feature is to send the moves of
> the losing side to IDeA and search for improvements that have not been
> analyzed before.

The important points here are:

1) "Generate Alternatives" replaces the tasks that would otherwise be sent to IDeA. They are not sent in addition to the normal tasks.

2) When a game is being sent to IDeA, the moves in that game will be excluded in the search for alternatives.

3) If you select "Generate Alternatives", but don't specify "Alt tree", only the moves in the game are excluded in the search for alternatives.

4) If you do specify "Alt tree", both the moves in the game and the moves in the tree are excluded in the search for alternatives.

Here is a simple example.

I open a database game and send the first two moves to an IDeA project: 1.e4 c5 2.Nf3 d6.

a) If I don't select "Generate Alternatives", the four positions after 1.e4, 1.e4 c5, 1.e4 c5 2.Nf3 and 1.e4 c5 2.Nf3 d6 will be analyzed normally in IDeA. This it the most common usage of "Send Games to IDeA".

b) "Generate Alternatives", but no "Alt tree": IDeA will search for alternatives in the four positions after 1.e4, 1.e4 c5, 1.e4 c5 2.Nf3 and 1.e4 c5 2.Nf3 d6. When it searches for an alternative after 1.e4 it will exclude 1...c5 from the search (since that is the move in the game) and look for another move. Similar for the other three positions; the move in the game will always be excluded from the alternatives search.

c) "Generate Alternatives" with "Alt tree" works similar to b), except more moves will be excluded in each position. When it searches for an alternative after 1.e4 it will exclude 1...c5 and any moves in the "Alt tree" from the search. If the "Alt tree" contained analysis of the French Defense, it would contain the position 1.e4 e6 and consequently both 1...c5 (from the game) and 1...e6 (from "Alt tree") would be excluded in the search for an alternative after 1.e4.