This is another example of terms, definitions or cues being slightly different from one hitting guru to another.
Bat
lag is a term Slaught uses to describe the "crack the whip" action of the rotational swing. The hips and core turn first, with the final movement of the wrists and bat being the last piece in the chain (ie: the end of the whip when you "crack the whip"
He is describing the transfer of rotational power through a mechnical chain that remains linked (connected) to the body's core.
Bat
drag is a term used to describe "slop" in the swing where the bat is lagging behind the rotation to a degree that you loose some of the "connection" with the body's core.
The article describes something I have also heard discussed in different terms- the difference between bat
speed and bat
quickness.
Bat speed refers to the maximum bat speed acheived. Bat quickness refers to the elapsed time from the point the swing is initiated to ball contact. They are not the same thing and there is a trade-off made to acheive one or the other.
For example, think of a drag race car as it leaves the line. One car might spin the tires before hooking up. Ultimately, that car might eventually hit a higher speed, but its elapsed time will be slower because the speed came later in the race than the other car.
Another example that relates more to a swing: Slow pitch softball players have been recorded with bat speeds in the 110-120 m.p.h. range. Yet Major League baseball players will top out at a bat speed of about 90 m.p.h. There has been a tradeoff between bat speed and bat quickness. The slow pitch player has all of the time in the world to react to the slow, arcing pitch. Bat quickness can be sacraficed to get a higher bat speed. The baseball player needs more bat quickness to allow more reaction time to changing pitch speeds and breaking pitches and some ultimate bat speed will be sacraficed.
Bat quickness is promoted through removing "slop" (tire spinning) in the rotational chain. This allows the hitter to wait a tiny fraction of a second longer to read the pitch and react accordingly. While some bat speed must be sacraficed, the advantage to the hitter being able to hold off a few more ticks will result in more consistent contact with the ball.
You can generate all the bat speed in the world, but if you can't do it quickly you will be reacting late to the pitch and decrease your chances of getting the bat on the ball.