Sending signals across domestic electrical wiring isn't a 21st century idea. In fact, the power companies have been sending control signals over the mains since the 1920s - it's how electricity meters know when to switch to an off-peak rate.
The electrical wiring in the average home can support a variety of frequencies. As electricity uses 50/60Hz signals, extra data can be transported along the same wiring at much higher frequencies, without causing any interference.

The original HomePlug 1.0 standard (IEEE 1901) operated at 14Mbps, although real-world speeds were closer to 5Mbps. A 'Turbo' upgrade boosted the maximum throughput of version 1.0 to 85Mbps (real-world speeds of around 20Mbps). In 2005, a revamped HomePlug AV boosted the rate even higher, claiming 200Mbps on the box (80-90Mbps in tests) - more than enough to cope with audio and video streaming, hence the 'AV' tag.

The older power-line technology used by Intelogis relies on frequency-shift keying (FSK) to send data back and forth over the electrical wires in your home. FSK uses two frequencies, one for 1s and the other for 0s, to send digital information between the computers on the network. The frequencies used are in a narrow band just above the level where most line noise occurs.

Although this method works, it is somewhat fragile. Anything that impinges on either frequency can disrupt the data flow, causing the transmitting computer to have to resend the data. This can affect the performance of the network.
For example, it is possible that using more electricity in the house, such as running the washer and dryer, the network will slow down. To help reduce the amount of electrical-line noise line-conditioning power stripscanbe used.

PowerPacket is an alternative technology that makes use of orthogonal frequency division multiplexing (OFDM) with forward error correction. This is a variation of the technology used in DSL modems and phone line networking.
Just as FDM uses a separate frequency on a standard phone line to transmit data by dividing the available frequency into distinct data channels and uniform bandwidth, OFDM utilizes the range of frequencies in an electrical wiring system and splits this into 84 separate chunks. OFDM then sends the data simultaneously through these carrier frequencies.

The operating basics of powerline technology is that a power source, giving 220 volts for example, operates at a rather low frequency of 50-60 Hz and the first few harmonics which means there is a massive chunk of the cable in the frequency domain that is being unused. So use is made of it to transmit digital data.
Connecting a capacitor with a low value and an inductance of a low value to this line will result in the low inductance not allowing 50-60 Hz to pass as it shorts such currents to ground. It is placed after capacitor so as not to short everything.