Magic, I dunno actually... I know that in any given situation if there's something nobody wants to do and nobody wants to make the first step, even if I hate it just as much as anyone I notice I tend to be the initiator most of the time... I think 'initiation' is something ENTP's kinda do naturally... it's a sorta 'oh ffs let's get this ball rolling!' feeling I know I, personally, have... so whatever 'that ball' is, all it takes is a feeling that if I don't, nobody will, and that's enough to motivate me even if it's something I hate to do. And especially when it's ENTP/INTP as a pair, I would imagine the E would make me by default usually looked to as the initiator.
TBH it's one of the gripes I have in life generally, that though most of the time I don't mind it, it sorta gets on my nerves that people seem to usually RELY on me being the one that 'gets things going'; people don't seem to even think about making the effort themselves once in a while...