Visama is the opposite of Sama (something equal or balanced).
In Sanskrit Visama means "irregular" or something is not equal.
So all 4 phases of pranayama are not in equal length in this technique (time and length wise).
The important thing here is the ratio. Ratio of inhalation (puraka), pause (kumbhaka), and exhalation (rechaka).
We can consider numerous variations of ratios. The ideal ratio is: 1-4-2-4 with both antar kumbhaka and bahir kumbhaka.
Example: if the time of inhalation is 8 seconds then exhalation should be 16 seconds & both kumbhakas should be 32 seconds.
But, to obtain this ratio practitioners required much time and patience.
Practitioner should make classic rhythm first only with antar kumbhaka that is 1-4-2.
For this, start with the ratio 1-1-2, then 1-2-2, then 1-3-2 then 1-4-2. This is the sequence to obtain the ideal Antar kumbhaka.
Only then, the practitioner can start working for the ideal ratio with bahir kumbhaka too.
For this, start from 1-4-2-1, then 1-4-2-2, then 1-4-2-3, then 1-4-2-4 ...
Boost the immunity/vitality.
Increase the efficiency of the lungs, heart and all those organs who participate in the digestion system.
Increase the level of the oxygen in the blood.
Relax the nervous system and calm the mind.
Practice in the guidance of experienced teacher.