John,

Basically you choose how many dice (of the total number available for that asset) you wish to "risk" on each attempt (i.e. each turn).

So, say you have chosen Tank Hunters, and for your force this has a value of 11. You could throw all 11 dice in one go, but then you might expect to get, on average, almost 2 6's. This could be seen as a waste of dice, as you could not try for this asset again (no more dice left).

You may quite like Tank Hunters to arrive on that turn, but not be desperate, and so choose 4 dice, leaving 7 left. You have a slightly-better-then-evens chance of getting a 6 from 4 dice and still get to have another go or two in later turns, each with a good chance of scoring a 6. Even leaving just 1 or 2 dice in reserve is handy, since your opponent can't be sure you won't score a 6 at that crucial moment.

Does that help at all?

Simon