Wichita State in the late 1970's, was looking for an "equalizer" to help them be competitive with schools that were bigger, faster, and stronger. The staff there sat down and came up with what we know as the midline option. Of course true triple option teams such as Georgia Tech and Navy run the midline option, but it is starting to be implemented by more teams in both high school and college that would not be considered true option teams.
Advantages of the midline:
1) The tight mesh of the QB/FB serves as a means to freeze the linebackers to give the offensive linemen a half step advantage
2) Forces DTs to close one of two gaps
3) Collapses the defensive front
4) Allows option football to be ran to the short side of the field
5) Allows a dominate DT to be "read" instead of having to be blocked
6) Hits quickly
7) Minimizes chance of lost yardage play
8) Easiest of all the veer reads
How to block midline:
The following two diagrams show how midline is blocked versus a 4-2/4-4 look. For information on blocking midline versus odd fronts, visit the different links in the resources section at the bottom.
As you see in the diagram above, the QB is reading the 3 technique DT for a give or keep read. If the DT gives him a keep read, the QB will then key the SAM linebacker. He will pitch the ball to the back if the SAM closes inside.
In this diagram, the QB's pitch key has changed from the SAM linebacker to the DE. As mentioned, this scheme takes advantage of an over aggressive OLB playing the QB. The pull and pitch on this play can be very "bang, bang", so repping the pull and pitch process with your QB is vital. At the 0:10 mark, you can see Navy running this exact scheme vs. Notre Dame.
Instead of using the remaining back as a pitch guy, some prefer to lead the back up on the outside linebacker/most dangerous defender instead of pitching off of him. This turns the play from a triple option to a double option. Here is a clip from The University of Charleston where instead of using the motion back as a pitch guy, he leads up for the QB carrying the ball.
The following two videos feature Muskegon (MI) High School and their midline game out of their spread pistol formations:
Hinsdale (IL) coach Mike DiMatteo explains their midline option from the gun on ChiefPigskin.com
Quarterback Mechanics and Mesh for Midline Option by Jerry Campbell
Flexbone Option Website Midline Presentation