Power Stacks can randomly add stacks of matching symbols across all five reels during ordinary play. In the base game, most regular symbols are eligible to become stacked, increasing the chance of creating larger line combinations across the 40 paylines.

It's a simple mechanic, but one that works particularly well on a four-row grid. A couple of large stacks lining up across neighbouring reels can transform a fairly ordinary spin without requiring another bonus to activate.

I also like that Power Stacks don't disappear once Free Spins begin. Instead, the game changes what they can contain, which makes the feature feel like an upgraded version of the same idea rather than an unrelated bonus bolted onto it.

Landing three, four or five Scatters awards 10 Free Spins, with Power Stacks remaining active throughout the feature. The important improvement is that only Wilds and Coins can become Power Stacked during Free Spins.

That's a much stronger symbol pool than the base game offers.

Instead of watching a large stack of relatively ordinary symbols appear, you're hoping those oversized blocks contain the two things most likely to make the bonus interesting. Stacked Wilds can dramatically improve line wins, while stacked Coins can help push the feature towards ULTRA Link&Win.

It's a sensible upgrade because the bonus doesn't need another complicated rulebook. It simply makes an existing mechanic considerably more dangerous.

Coin Symbols provide the Cash prizes used by ULTRA Link&Win. Landing six or more Coins activates the feature, with their individual values carried onto the bonus grid.

Coins become particularly interesting during Free Spins because Power Stacks can produce them in much larger groups. That gives the two features a natural connection rather than leaving Hold and Win completely separate from the rest of the game.

A big stack of Coins therefore isn't merely attractive because of the values printed on them. Six is the number that opens the temple door.

Landing six or more Coins activates ULTRA Link&Win, All For One Studios' enhanced Hold and Win feature.

Coins remain locked while the empty positions Respin, with new qualifying symbols keeping the feature alive. What makes this version considerably more interesting is the collection of Booster Coins that can manipulate the prizes and even increase the size of the playing area.

I like that because it prevents the feature becoming a straightforward exercise in filling spaces. Another Coin is useful, but the right Booster can be considerably more important depending on how the board has developed.

The feature starts recognisably enough and then gradually becomes much busier as the Boosters get involved.

The Persistent Cash Collect gathers the values of all eligible Cash Coins on every subsequent spin until ULTRA Link&Win ends. Only one persistent Collector can appear during a feature.

This is easily one of the strongest Boosters because its value grows alongside the board.

Get it early and every new Cash Coin has the potential to be collected repeatedly as the Respins continue. If the feature subsequently expands and fills with stronger prizes, that same Collector becomes increasingly useful without needing another one to land.

It's the sort of symbol that can make you suddenly much more interested in keeping the volcano erupting for another few spins.

The standard Cash Collect Booster immediately gathers the values of eligible Cash Coins already present on the grid. Up to three can appear during a feature.

Unlike the persistent version, this is a one-off collection, so timing matters considerably.

Finding one when only a couple of modest Coins are visible won't change much. Get one after several rows have filled with worthwhile prizes and it becomes a much more welcome arrival.

The Persistent 2x Multiplier randomly doubles the values of up to four eligible Cash Coins on every subsequent Respin until the feature ends.

This is another Booster that becomes better the longer the feature survives because it can keep improving Coins rather than applying its effect once and disappearing.

There's an interesting restriction too: a feature can contain either the Persistent Cash Collect or Persistent 2x Multiplier, but not both.

That's probably wise. Letting both loose on the same successful grid might cause an eruption that even the volcano wasn't insured for.

The regular 2x Multiplier randomly doubles up to four Cash Coin values for the current spin, with as many as three of these Booster Coins capable of appearing during ULTRA Link&Win.

It's naturally less exciting than the persistent version, but several can still substantially improve a board that's already filling nicely.

I like the distinction between the two because landing a persistent Booster feels like a genuine upgrade rather than simply receiving the same modifier with a slightly different animation.

The Unlock Booster opens an additional row during ULTRA Link&Win, with as many as three extra rows capable of becoming available.

This is probably my favourite mechanic in the feature because it physically changes the amount of game available to you.

Every additional row provides more positions for Coins and Boosters, giving a strong feature more room to develop rather than gradually suffocating as Sticky symbols occupy the original grid.

There's something particularly satisfying about a bonus where success gives you more places to succeed.

Extra Chance provides another Respin after the normal counter has expired, giving a promising ULTRA Link&Win feature one final opportunity to continue.

It isn't as glamorous as unlocking another row or landing a persistent Booster, but its usefulness becomes painfully obvious when the board is already full of good prizes.

Hold and Win features have an extraordinary ability to produce their final blank just when things start getting interesting. Extra Chance gives the volcano one last opportunity to cough something up.

King Millions Tokens can appear on reel five during base play, with a maximum of one Token available per spin. Collecting them provides the chance to trigger the King Millions Bonus.

The Tokens themselves don't provide ordinary line payouts. Their entire purpose is feeding the jackpot mechanic, which keeps them visually and mechanically separate from the Cash Coins used by ULTRA Link&Win.

That distinction helps because this is already a busy game. Coins build one feature, Tokens chase another, and you quickly learn which sort of treasure you're looking at.

Triggering King Millions opens a wheel containing cash prizes between 7x and 100x the stake, alongside the Major and Grand progressive jackpots. The progressive prizes are funded through the wider jackpot pool and reset after being won.

I like the fact that King Millions is clearly separated from ULTRA Link&Win. The Coin feature has enough going on without trying to incorporate a progressive jackpot into the same grid.

The wheel also gives those otherwise non-paying Tokens a very obvious destination. You're not collecting them because another meter happens to need filling; you're building towards a completely different shot at the game's largest prizes.