Distributed Coordination Function (DCF) of IEEE 802.11 WLANs prohibits concurrent transmissions to avoid packet collisions or interferences. This conservative transmission mechanism of DCF makes it hard to fully utilize the wireless capacity in terms of spatial reuse. Recent measurement studies demonstrate that a packet can survive the collision and can be decoded successfully by the receiver depending on the transmission order and relative signal strength. We call this physical layer Capture Effect. A new wireless PHY technology, namely, Message In Message (MIM) augments the probability of the PHY capture by adopting the enhanced preamble detection function. In this paper, we propose an opportunistic concurrent transmission protocol which exploits MIM functionality to maximize the system throughput of IEEE 802.11 WLANs. The key idea of our approach is that when an AP overhears other APs' transmission, it sends a packet to its client concurrently if the MIM capture threshold requirements are satisfied. Our ns-2 simulation results show that our scheme outperforms DCF up to 18% in most cases.