I have one of these in my car.
Once it gets to the lock speed, it will stay locked until you get under the unlock speed, about 5 mph below the lock speed setting. It doesn't care what gear your transmission is in or if it kicks down or you shift it or whatever.
One thing to note. I found that the output voltage wasn't high enough to keep the solenoid engaged. I had 2 boxes that went bad, and would only put out 6 volts. I finally ended up having to feed the output to a small relay and use 12 volts from before the brain box as the relay output.
When I'm driving, and need to accelerate at a moderate level from a low cruise RPM, I like to (read need to) unlock the converter and allow the engine to rev up, not lug. For this, I installed an adjustable vaccum switch. Mine is set at 7" hg, so once I press the pedal far enough, (but not enough to kick down to 3rd) it will unlock. Once I let up on the gas, it will relock. I couldn't get by with just a vacuum switch though, or it would chatter in and out if I was just at that 7", or it would relock immediately when you let off the pedal. To solve this problem, I installed a vacuum delay valve before the adjustable switch. This will cause a several second delay once you re-establish your cruise level and stop it from kicking in and out.
I found this to be the best way to make it work like I felt it should. With the vacuum switch, it makes sure that I'm never locked under heavy throttle, which makes the car drive much nicer. I lock mine at 55 MPH, and it unlocks at 50 ish. I just wanted the B&M to stop things from locking until I got up to highway speeds. I run a deep gear (4.56) so I'm in 4th very quickly buy never want it locked under 2000 RPMs (which is 55mph for me).
I also have a switch I flip that bypasses all this stuff and allows me to lock it with a seperate switch in any gear. Then, when I'm draging, I hit the lockup in 3rd gear and let the engine work down the top of the track. It does make it faster in the 1/4 mile. I have a heavy duty clutch converter designed to do this however.