It is part of the proportional representation set up in the D'Hondt electoral system that is used for Scottish elections. Briefly what happens is that in many Scottish seats the dominant party (1st vote for constituent seats), i.e. the SNP, can have its massive list vote (2nd vote) significantly diluted (can't remember the % reduction offhand) because that political party is the dominant one. So a second placed lesser party with 6% of the vote can win a list seat to the detriment of say a huge 2nd vote for the SNP (unless that reduced SNP vote is still too big to overhaul) and to the chagrin of many SNP voters who regard their 2nd votes to be utterly wasted esp when certain Tory and Labour list candidates are picking up a Scot Parliament ticket (with a tasty annual salary of about £70k) and are unelected by the people. Hence many SNP/indy voters have debated how not to waste their second vote. In other words voting for another popular indy supporting lesser party like eg the Green Party whose vote will not be diluted. The difficulty with this kinda move is that one can't predict what indy party is the one for every SNP/indy voter (in each voting region) to plump for with certainty of the dominant vote not being wasted? IMO it is not certain that 1000s of indy supporters will opt for the Alba Party. At the moment lotsa SNP/indy supporters would love to wipe out as many unelected Unionist MSPs as possible who they regard as unelected "back door carpetbaggers" with no interest in Scotland or Scotland's future and the way forward away from Brexit and a hostile Brit govt who we never voted for AGAIN. Alex Salmond is clearly looking to capitalise on that position but the fear for many is that he may well dilute the vote in some regions where the list vote is still won by the SNP. The clear objective for most indy supporters is to ensure there is an SNP govt in place to ensure Johnson takes notice that it is a big enough govt to push for Scot indy. Any additional Scotindy MSPs would be welcome and regarded by many as true justice imposed for the second list vote and hopefully there will be far less "back door" Unionist MSPs picking up big salaries after being picked from the top of their own party lists. In the case of the Scot Tories, some of their top list candidates are people who are ex party donors who have limited interest in Scottish affairs - a f##### joke!!