On an electrified railroad, section breaks are gaps in the overhead wire used to enable sections of it to be switched off, for maintenance among other reasons. In normal operation the sections are joined electrically.
Obviously a simple gap is not going to work; the locomotive's pantograph, sliding along the contact wire at maybe over 100 mph, would just catch in a gap and pull down the overhead as well as wreck itself.
Instead, the two wires are exchanged over a span of multiple catenary poles, so that the pantograph is always in contact with a wire. The new wire starts off attached to the actual pole rather than the cross-brace, because the contact wire (trolley wire) is under considerable tension, tight enough for people to walk on; the pole is tougher. The wires start off higher than the current contact wire but drop towards the second catenary pole, where they start to take on the standard catenary form. Between the second and third poles the new wires slowly drop, while the old wires slowly rise; thus, between these two catenary poles, the locomotive's pantograph starts to run along the new contact wire rather than the old. After the third catenary pole, the old wires veer off to the side to tie off to a catenary pole directly, in exactly the same way as the new ones began.
Thus, there is a smooth transition from the first to the second section without the two sets of wires touching at any point.
With help from Mark Bej's The Broad Way at http://broadway.pennsyrr.com/