FYI I split this out into it's own question from your other question about avoiding scratching so much.
Regarding the advice from Dr. Dave:
[...] energy is lost with each collision of balls, and that if the rack is perfectly tight there is only one collision during the break.
I think I can explain what he means.
Energy transfers from your arm to the pool cue, from the pool cue to the cue ball, and from the cue ball to the first object ball it hits in the rack, and from that first object ball to each other ball ahead of it, and so on.
A little energy is lost at each transfer. A little is lost while the cue ball is traveling, and a little more is lost when it hits the first object ball.
If the object balls are all perfectly tight and touching, it's as if the energy transfers from the cue ball to the entire rack of object balls as though it was a single object, thus, no energy loss during the slight travel and subsequent collisions with one or more other object balls.
If the object balls are NOT perfectly tight and touching, then each tiny amount of travel that each object ball takes is an opportunity for energy loss. Then each object ball collides with one or more other object balls immediately adjacent to it, and in each of those collisions some more energy is lost.
With 15 object balls in the rack, consider that if each one travels a bit, that's 15+ tiny losses of energy. Then consider that each ball will hit 2+ more object balls... e.g. the first ball struck will hit the two in the next row of balls in the rack. Each of those will hit 2+ more object balls in the next row, and so on... That's 20+ small collisions during the break shot which cause energy loss.
Not sure if that's how he meant it, but that's how I envision it.
I think that's what they are saying in the AZB thread where the original poster asked the same question as you've posed here.
Here are the main answer highlights from that question (in my opinion):
User garczar said:
[When there are gaps between object balls in the rack] the transfer of energy is interrupted leading to funny results. The cue-ball is NOT applying same force when there are gaps. Gaps also create angles within the rack that completely changes the way they open up.
User BBB said:
[When there are gaps between object balls in the rack] the forces are not spread evenly [as it transfers from ball to ball in the rack]
User straightline said:
A properly frozen rack is supposed to act like a solid. Probably the initial impact is transmitted throughout [the solid] then explodes as internal stresses try to escape. If the rack isn't frozen, the transmission will be haphazard and the latency and resultant caroms will be commensurately haphazard and unpredictable.
So it seems to me that there are two primary reasons you would want a tight rack for a break shot in pool:
- So that the racked balls break in a consistent way (if they are lose, the object balls will break in an inconsistent way, which is undesirable to the person breaking).
- So that the maximum amount of energy coming from the cue ball is retained and used in spreading out the pack of object balls as such that the breaking player can have an easier time running the table.