A question that's popping up more and more..
Effectively, "strapping" does electrically the same thing as "bridging" does inside a stereo amplifier. It references the negative voltage rails of the two different amplifiers, and then inverts one of the positive input signals, allowing the load (the speaker) to be connected across the positive voltage rails. This also eliminates the need to match multiple gains, filters and crossovers. Simply set the 'Master' amplifier to your preference and the 'Slave' amplifier will produce the exact same signal and power. Also keep in mind that strapping doubles your power at twice the impediance load.
Example: Two amplifiers each capable of 2000 watts @ 1 Ohm will do 4000 watts @ 2 Ohms strapped, essentially making them one mono amplifier.
Not every amplifier has onboard strapping capabilities but can still be strapped with linking devices such as Maxxsonics Maxxlink MLX-100. Here's an informative video on how to use this nifty little device.
Here's a basic diagram I made that shows how to tackle the wiring for onboard linkable amplifiers. Be sure to set any switches to the correct "Master/Slave" position.
Hope this helps those who are confused about amplifier strapping.