Warren,
The Sear is the extension on the firin pin which is engaged by the Sear Bar, and disengaged to fire the pistol.
Take you Luger apart, and remove the toggle train from the receiver rails. In the illustration below you see a breechblock with the firing pin inserted; the Sear is the block on the firing pin which sticks out the slot on the left side, and is pointed out here by the blue arrow.
In the illustration below you see the Sear Bar in the left receiver rail, outlined in red.
Press inward (toward the center of the pistol) on the area pointed out in blue, you will see that the bar pivots on its center. If you look at the inside of the receiver rail, you will see that when you press inward, the back edge moves outward. This edge is what actually engages the sear on the firing pin.
Reassemble the toggle train into the receiver and close the breech.
CAUTION! Make sure there is not a cartridge in the barrel!!
Press inward on the front of the sear bar again. This trips the sear, and the firing pin fires.
You can see from this demonstration that it is possible to fire the weapon while it is disassembled if there is a shell in the chamber.
There is a small, cylindrical piece of metal in the front of the sear bar. Press it into the sear bar; you will see that it is spring-loaded. This is the disconnector.
Reassemble the receiver into the frame, and replace the trigger plate. As you do so, note the relationship of the pivoting, L-shaped piece of metal (trigger lever) in the trigger plate to the trigger and the sear bar.
As the trigger is pulled, it pulls down the leg of the trigger lever (red arrow), which pivots the top of the trigger lever (blue arrow) inward. This presses inward on the cylinder at the front of the sear bar, thus tripping the sear bar from the firing pin sear and firing the weapon.
As the receiver moves to the rear under recoil, the disconnector moves backward, out from underneath the top of the trigger lever. As the next cartridge loads and the receiver moves forward into firing position, the trigger lever presses the disconnector into the sear bar, making sure the firin pin sear is engaged with the sear bar and cocking the gun.
When the trigger is released, the disconnector springs out of the sear bar, into place under the trigger lever, and the weapon is ready to fire again.
I hope that this is clear enough, with the actual parts in your hand.
--Dwight