As to the siphon question, siphon's are formed by gravity. The reason they work, is because the gravity forces water to fall through a closed system (in this case a tube.) Once you get it over the back of the tank, the force of gravity forcing it through the end of the tube or hose is greater than that pulling it back into the tank (note that siphons only work when the end is LOWER than the beginning). As this happens, the siphon creates a low pressure point (vacuum) where the water used to be, just like a straw. This forces more water into the tube, and it keeps perpetuating itself. Once air enters a siphon, it changes the pressure, and gravity will pull the water in the tube down, instead of through, so the water behind the tank in the u tube still goes down, but the water inside the u tube in the front part of the tank goes back into the tank, and your siphon is dead.
The reason an overflow box with two compartments, or with 1 and a standpipe work, is even when the power shuts off, there is still water in the one compartment, or in the whole box higher than the level of the u tube. The water level INSIDE the inside compartment of the box reaches the same level the box behind the tank, giving both equal pressure. The siphon stops, but doesn't break, because there is no air, it's just that the pressure is the same, so it reaches equilibrium. (water has stopped spilling over the inside overflow box, but it doesn't lower enough either, because in a well made overflow box, the water level on the other side of the system will be higher than the input end of the u tube also. That's why most U tubes you see have very little difference in which end is the high end and which is the low. Power comes back on, adds water to the inside box as it floods, and that increases the pressure on the inside box relative to the outside box, and the water naturally flows from high to low again, through the u tube.
One thing you have to worry about is getting too many bubbles collecting in the top of your u tube that they will actually break the siphon by themselves. This isn't common, but all it takes is once, and you'll really be in trouble unless you catch it quickly. The aqualifter fixes that problem, because it pumps all the air out of the top of the u tube, ensuring that enough bubbles could NEVER collect enough to break the siphon.
This would be so much easier if I was holding one in my hands, and even then it took me about 2 hours of staring at an overflow box before it made sense to me, so this may not help, but if nothing else, remember that air kills a siphon, and water (like everything else) moves from high pressure areas to low pressure areas.