What do you mean by sensitive slicers?
In this context, a slicer is something like a combination of a comparator and a D flip-flop. 

The comparator functionality means that it can very accurately detect whether the input is above or below some threshold. Often, the threshold is adjustable. 

The flip-flop functionality means that the comparator result is captured at a very specific instant in time, according to the clock input of the circuit.

In the case of the receiver for a digital transmission system like PCIe, some algorithm would be used to optimize the threshold level and the timing of the clock signal to minimize the error rate of the system.