What is Aachamanam?

Aachamanam - Telugu dictionary(SriSabdaakaram) defines Aachamanam as the style of sipping water 3 times by pointing your "pointing-finger" upwards and taking water in your palm.

The general mantras that go with Aachamanam are (3 sips with three names of Lord Vishnu )

Om Kesavaaya Swaaha
Om Naarayanaaya Swaaha
Om Madavaaya Swaaha

Followed by cleansing your hand with little water uttering "Om Govindaaya Namaha"

"Aachamaneeyam" is generally followed by the upachaaraas that have to do with WATER like Paadyam(Washing Swami's legs with water), Arghyam(Washing Swamy's hands with water) and Snanam(Holy bath for Lord). I think this is a way for us to meditate that the water that Swamy has used becomes holy(Tirtham) with his touch and we sip that water for Aachamanam.(I need to confirm this by talking to a purohit)

The following are the rules that need to be observed while performing
aachamanam:

1. One should sip water "3" times to perform aachamanam

2. The amount of water taken for sipping should be only the size of pea.
One should not consume more just because it feels comfortable or quenches
the thirst.

3. Water that contains foam/bubbles/suds ("nurai" in Tamil) is unfit for
aachamanam.

4. aachamanam should be performed only with cold water and not with
warm/hot water even if it is uncomfortable to the throat.

5. Scents/flavors should not be added to the water meant for performing
aachamanam.

6. aachamanam should be performed only in sitting posture and should not be
done standing.

7. One should not perform aachamanam facing the south.

8. aachamanan should not be performed in unclean places.

Source: "kurai onrum illai" (pp 277) in Tamil by Sri Mukkoor Lakshmi
Narasimhaachaaryar.