My partner, a very experienced player, made what I thought was an odd call the other night in an IMP pair game.
With this hand, first, he made a 2/1 (which is not game forcing if he rebids his suit) and then leapt to 5♣ over 2NT. I dug a deeper hole for us when I decided that 5♣ could not be natural (the only interpretation that made some sense to me was exclusion RKCB for hearts), since he could have bid 3♣ and then 4♣ or 5♣. He said his 5♣ was intended as "fast arrival."
Any comments?

Help

1♥ - (P) - 2♦ - (P)
2NT - (P) - ?