Shri Chitravina Narasimhan gives an explanation of the ancient marga talas which consisted of only 3 angas, laghu, guru and plutam.

Here is a summary:

Marga Talas were the precursor to talas that exist today. They included only 3 angas, namely  Laghu , Guru and Plutam. Although we use many more angas today, like the shadanga, shodashanga and samyuktangam , only 3 were used in those times. Laghu by default means a count of 4. Guru is a count of 8 and Plutam is a count of 12. They included the 5 talas Chachatpuṭam ,Chāchapuṭam , Shaṭpitāputrikam , Sampadvēṣhṭakam and Udghatam . These were used in the Prabhandams which were ancient religious Hindu texts.

The guru explains that the name of the tala has been expressed as a rhythmic pattern to help us remember the angas contained in that tala. As we saw earlier in Part 1 and Part 2 of this series, a short syllable ‘A’ represents a Laghu, a longer syllable ‘Aa’ represents a Guru and the combination results in a Plutam.
For Example in Chachatpuṭam, it can be said as Cha. cha. pu tam.. which is 1 Guru + 1 Guru + 1 Laghu + 1 Plutam which results in 8 + 8 + 4 + 12 = 32 Aksharas.
The same idea is applied to all the other talams as well.
To render a laghu, one has to start with an audible beat (shashabdha kriye) followed by 3 counts of the fingers which are silent. (Nishabdha kriye)
To render a guru, one has to start with an audible beat (shashabdha kriye) followed by 7 beats which are shown as movement of the closed fist from left to right in clockwise fashion. (Nishabdha kriye)
A plutam consists of 3 parts, a kaadai, hrishya and sarpini where each part represents 4 beats adding up to 12 in total. A kaadai is one audible beat followed by an upward palm for 3 inaudible beats. The Hrishya is rendered as the palm facing inward to the left for 4 inaudible beats(Nishabdha kriye) and sarpini is palm facing outwards to the right for another 4 inaudible beats.(Nishabdha kriye)