Bet spreading refers to the difference between your lowest and highest bets in blackjack when you are counting cards. When you count the cards and you know there are more low-valued cards left to be played, you can bet minimum. When you know that there are more high-valued cards remaining, you can bet maximum.
Now that is an over-simplification. Card counting isn't a 0 or 1 thing like computer language. The count goes up and down in steps by 1 point at a time. On the average, player edge increases by about 0.5% with every point increase. It is said that card counters should increase their bets in proportion to their advantage.
Problems with Bet Spreading
if you were to bet maximum every time there is a player's edge, no matter how small it is, you are risking a lot of your money. Why? Because you will have to "max bet" more often.
If you can afford it, you can get a bigger session bankroll for this. Instead of a bankroll just 50 times your minimum bet, get a bankroll that is several hundred times the minimum bet.
By the way, the minimum bet must always be the same as the one for the blackjack table you are playing at. The maximum bet must not exceed the one at the table.
To protect your bankroll, you have to have max bet less often. Before you max bet, make sure that the odds justify it. And a small player's edge does not justify a big bet, does it?
Another problem with this "min/max" betting is it is an easy tell for casinos. If you always bet minimum when the house has the edge and maximum when the player has the edge, they will know you are counting cards and ask you to leave. It is very suspicious indeed for a player to always bet either $5 or $40 on every hand!
Bet Spreading System
To avoid detection and to protect your bankroll, you should step up (or down) or your bets along with your count. You can devise your own; you can put any cap on your maximum bet so long as it does not exceed the maximum bet at the table. Here is an example:
True Count Your Bet
0 or less $5 1 $5 2 $15 3 $25 4 $35 5 $45 6 $60 7 $80 8 $100 9 $100 10 $100
The bet here is capped at $100, so no matter how high the count gets, you will always bet $100, *usually.* Sometimes though, you may want to bet higher or bet or less according to this table to avoid detection. If your bets get predictable, the casino folks will see what's going on. So be careful!