Today the free spins bonus round has become an integral part of online slots. The latest online slots like Immortal Romance have complex free spins structures in which a number of other features are also combined. However, this was not always the case. In the earliest free spins slots when the triggering mechanism appeared on the reels (usually three or more scatter symbols) the stated number of free spins were automatically played. In many online slots there was a provision that the free spins would retrigger if the required mechanism appeared on the reels during the free spins. The first enhancement to the free spins was the multiplier. The payouts during the free spins were in stated multiples of the normal payout tables.

Originally the rules of the online slot game laid out the number of free spins and the multiplier. Later, interactivity was introduced in specifying the parameters. Playtech has a slot game titled A Night Out with the theme of girls partying. When the free spins are triggered the player selects a girl and the number of free spins is indicated by his choice. Then the player buys the girl a drink and the multiplier is again indicated by his choice. Actually the parameters are randomly decided, but the animated process that plays out on the screen makes it more interesting. In some online slots the player actually determines the parameters. He is given combinations of number of free spins and multipliers and has to pick one of them. When the number of free spins is large the multiplier is small and when the number of free spins is small the multiplier is large.

One of the complexities introduced in contemporary free spins is a counter. When the triggering mechanism appears on the reels then the free spins are not immediately initiated. The counter increases by one. When the value of the counter reaches a given value then the free spins are triggered. A slot game from Microgaming titled Scrooge combines two counters. When the triggering event takes place either one free spin is added to the free spins counter or one day is subtracted from the countdown counter. When the countdown counter shows zero, the number of free spins in the free spins counter is awarded to the player.

The online casino software developer Realtime Gaming has introduced two innovative features connected to free spins. Free spins are triggered through random events and it is possible that hundreds of spins are played without triggering the free spins. In order to compensate for this bad luck Realtime Gaming has introduced Feature Guarantee in some of its slots. If the free spins round is not triggered for 150 spins, then it is automatically awarded to the player. Free spins are not triggered that often and when they are triggered then players hope to cash in. Nothing is more disappointing than a poor run in a free spins round. Therefore Realtime Gaming has introduced a Win-Win Feature that awards compensatory payouts to players faring badly in free spins.

Some slot games have free spins that can be terminated prematurely. In the Realtime Gaming slots game Hockey Hero the free spins end prematurely if two clock symbols appear simultaneously on the reels. The slot game Wipeout from WagerWorks has an even more interesting mechanism. The free spins begin with three lives. The Extra Life symbol adds one life and the Wipeout symbol reduces one life. If the number of lives becomes zero then the free spins round terminates prematurely.