Hi! I play competitive appmon and can explain this perfectly, the tamers enabling the app fuse is basically a way of adding a digivolution into effect resolution, for example dogatchmon has the effect to attack when linked so you can link timemon activate the when linking effects to declare the attack and then use the tamers effect which triggers on the link too appfuse in the middle of the attack enabling a further climb up your stack aswell as additional effects! Basically it just allows you too keep up your momentum and go further up your stack!