Just like lifelink, proliferatelink doesn’t use the stack and can’t be responded to. It’s just part of the results of damage dealt by Phyrexian Seedling.

Because proliferatelink lets you proliferate before checking for lethal damage on creatures, Phyrexian Seedling (and other creatures you control with +1/+1 counters on them) may be able to survive combat damage that would otherwise be lethal. For example, if a Phyrexian Seedling with one +1/+1 counter on it attacks and is blocked by a 1/1 creature, it would deal 1 damage to that creature and take 1 damage from that creature. Before checking for lethal damage, the Seedling’s controller would proliferate one time and could choose give the Seedling another +1/+1 counter. Then when state-based actions are checked, the Seedling would be a 2/2 with 1 damage marked on it and would survive.

To proliferate, choose any number of permanents and/or players, then give each another counter of each kind already there.

Each time you proliferate, you can choose any permanent that has a counter, including ones controlled by opponents. You can choose any player who has a counter, including opponents. You can’t choose cards in any zone other than the battlefield, even if they have counters on them.

You don’t have to choose every permanent or player that has a counter—only the ones you want to add counters to. Since “any number” includes zero, you don’t have to choose any permanents at all, and you don’t have to choose any players at all.

If a player or permanent has more than one kind of counter on it, and you choose for that player or permanent to get additional counters, it must get one of each kind of counter it already has. You can’t have it get just one kind of counter it already has and not the others.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.