One of the key areas of study in organic photovoltaics is the development of so-called 'nonfullerene acceptors’ (NFAs), which enjoy several benefits over older, fullerene-based acceptors, including: low open circuit voltage losses, low cost, high absorptivity, and broad tuneability. However, these advantages come at the expense of light harvesting efficiency. Recent reports demonstrated conversion efficiencies up to 17% in donor acceptor blends of conjugated polymers and ‘small’ electron accepting molecules. In order to understand the processes underlying this remarkable efficiency, we performed ultrafast transient absorption and optical control (pump-push and pump-push-probe) studies on the broad range of donor:acceptor blends. For many systems we observe that relatively long-lived excitons form at the acceptor regardless of whether the donor or acceptor is pumped, suggestive of rapid energy transfer from the donor to the acceptor followed by slow charge separation. A distinct change in exciton decay characteristics was observed at longer timescales in tandem with spectral drift in the acceptor localised excitonic peak. We use global analysis methods to elucidate the mechanism behind this feature as well as 3-pulse ”Pump-Push-Probe” spectroscopy to track the dissociation of interfacial charge transfer states within the material. Our results will help to shed light on the remarkable efficiency of this material and aid the development of more efficient and effective non fullerene acceptors.