Forum:Special Summon Effects eg Dark Necrofear

Dark Necrofear's effect is "This card cannot be Normal Summoned or Set. This card can only be Special Summoned by removing from play 3 Fiend-Type monsters in your Graveyard. If this card is destroyed by battle, or by your opponent's card effect while it is a Monster Card, equip it to a monster your opponent controls during the End Phase of this turn. Take control of the equipped monster."

Now, the bit I'm interested in is the line saying "This card can only be Special Summoned by removing from play 3 Fiend-Type monsters in your Graveyard."

Can this be done at any time, for example, if Dark Necrofear isn't in your hand (in your graveyard, deck etc) or during an opponent's turn or can it only be done during certain circumstances?

I'm basically confused over this whole concept, so any help is appreciated.