It doesn’t spam anything, it toggles the ‘down’ state of the key. So basically, when you hit (in this case mouse4) it’ll check whether the key in up or down state, and toggle that.
So it’s equivalent to holding down the W key. Not spamming it 
Hence it makes a good autorun replacement for games that don’t have it.
Same goes for the other one, which holds down mouse1 for me, which is very useful for games that want me to dig an awful lot while holding down a mouse button, which will pretty much leave me with extremely painful joints. So a toggle to turn on/off holding down the mouse button makes my games playable.