How Does it Work?
A bridge tied load (BTL) amplifier applies a normal signal to one terminal of the speaker, and an inverted signal to the other. If a single amp is capable of producing 20V RMS across the speaker, this equates to P = V2 / R, so in this case, 20^2 / 4 = 100W.
When connected in BTL, the same speaker "sees" 20V at one terminal, and an inverted 20V signal on the other - a total of 40V RMS (I shall leave the proof of this to the reader
Using the same formula, 40^2 / 4 = 400W - four times the power. But ... each amp now sees only half the load impedance (think of an imaginary centre tap in the voice coil, connected to ground). The amplifier must be stable into 2 ohms, or this method will not work. Of course, you can use an 8 ohm speaker and still get 200W if the amplifier cannot drive 2 ohms safely.