No problem, this isn't easy to pick up on. Let's say you Probe have just 3 things you want to Avoid. #1 is weekends, #2 is all nighters, and #3 is early show times. That's all you care about, so you could start out with bid group 1 by doing:

Bid Group 1:

Avoid SA-SU
Avoid All Nighters
Avoid Early Show times
Award Work L--

Let's say this bid puts a total of 30 trips (the yellow highlighted number) in your L-- pool.

The above can only be awarded if the system can honor all three of your Avoids. All the trips described by those Avoids are locked in the supply pool. As far as PBS knows, those trips don't even exist. Of course, you need to start loosening up your next bid group since there is a good chance that first bid group could fail (unless you are super senior) since there are only 30 trips in the pool. So let's say of your three Avoids, the first one you are willing to compromise on is the early show times:

Bid Group 2:

Avoid SA-SU
Avoid All Nighters
Award Work L--
Avoid Early Show times
Award Work L-

Let's say now that the trip pool (yellow number) shows 70 trips.

What you have done now is put Early show times "in play." Before, Early shows were totally locked out like SA-SU and all nighters. Now, they are availiable BUT they are frozen in the L-- pool. That means that even though you have made Early Shows available to the system now, they are stuck in the L-- pool which is the last place PBS will go looking for trips. If you looked at your trip pools, you see L-- and L-. L-- has the early shows, and L- has the stuff you like to do. By doing this, you are hoping that if PBS has to give you Early Shows, it will minimize the number of times it will do it. So, maybe you only get one Early show all month, instead of several.

Now you need to keep loosening up your bids in order to keep expanding your trip pool. So the next thing you say that you will give in on is All nighters. So repeat the above process like this:

Avoid SA-SU
Award Work L--
Avoid Early Show times
Avoid All Nighters
Award Work L-

Let's say now the total trips (yellow number) is up to 100.

Again, you have now put All nighters "in play". Up until now, they were totally locked away in the supply pool and therefore unavailable to PBS. Now they are available to be assigned to you. However, you have frozen them in the L-- pool along with the Early show times. Remember, as PBS attempts to process this bid, it will build as much of your line from L- as it can FIRST. Your L- pool has no All-nighters, No Early Shows, and no SA-SU. You want the system to build as much of your line as possible from L- before it dips down into the L-- pool where the stuff you would rather not do resides.

Now you can do your final bid group, which puts every trip in the base in play, but again keeps the stuff you really don't want to do in the L-- pool:

Final Bid Group:

Award Work L--
Avoid SA-SU
Avoid All Nighters
Avoid Early Shows
Award Work L-

Total trips available in pool let's say is now 150 (yellow number), which is every trip in the base.

This bid group can not fail. If you click on the yellow number and view your two pools (L- and L--), you would see the trips described by your 3 Avoids in L--, and all the rest of trips (what you prefer) in the L- pool. That way you are telling the system to give you the stuff that you prefer first before you try and give me any of the stuff I don't want to do.

As you can see, by simply dropping one Avoid at a time from the top of the list down to between L-- and L- with each bid group, you slowly expand the trip pool. That way, it is a negotiation with the system where you put Avoids "in play" one at a time to gradually expand the trip pool. Also, as you put those Avoids in play they always reside in the L-- bucket, so the system knows those are your last choice trips.

Next time the bid window is open, try messing around with it like this. It is easier to see what it is doing when you have it right in front of you.