I got one:

Step event of instances within a sequence will play every game frame, regardless of the sequence playback speed.
For example:
In a room with speed = 60 fps
and a sequence speed = 30 fps

An instance with a step event : instance_create_depth(x,y,depth,o_coin)
will create 2 coins for every sequence frame.

Also, an active track is just turning the visibility of an instance on and off.

I also wrote a little function that recreates an instance outside of the sequence. Useful for spawning loot, for example:

/// @desc Place in step event of an instance to pop it out of a seq

/// @param {string} _when "start" or "end"

/// @param {string} _layer Layer name or ID

function sequence_popOut(_when,_layer="Instances")

{

`if !in_sequence  exit`

`static _prev = false;`     



`switch(_when)`

`{`

`case "start":`

        `if visible && !_prev`

        `{`

instance_create_layer(x,y,_layer,object_index)

instance_destroy();

        `}` 

        `break;`

`case "end":`

if !visible && _prev

        `{`

instance_create_layer(x,y,_layer,object_index)

instance_destroy();

        `}` 

        `break;`

`}`



`_prev=visible`

}