Buffered bypass is the technique of designing a high input impedance amplifier to go ahead of the actual effect circuitry. The input impedance of this buffer is high enough to avoid tone sucking, and it has an output impedance low enough to drive the rest of the effect with no loading even if the effect proper has a low input impedance. This also allowed electronic switching from dry to effected signals, and became a favorite of several Japanese electronic companies. The best example is probably the Ibanez xx-9 series, which includes the fabled Tube Screamer 808 and 9. These effects do not get a lot of reports of tone sucking, although purists sometimes demand that they be converted to true bypass.
True bypass is the standard of clean signal quality against which all other bypasses are measured.
True bypass means that when the bypass switch is in the bypass position, the effect circuit is entirely disconnected, input and output, from the guitar's signal, and that the signal passes from the effect input to its output going only through wire and switch contacts. For this, we only need three simple switches and some timing. Clearly we need two switches to break the path from the input jack to the effect input and from the effect output to the output jack. If these two are simultaneously opened or closed, they connect or disconnect the effect from the jacks. It is actually possible to make such a switch, and it would serve very well.