The so-called l- sequence is: 90(x) - - 90() For a peak which is on resonance

The so-called l- sequence is:
90°(x) - τ - 90°(ϕ)
For a peak which is on resonance the sequence does not excite any observable magnetization. However, for a peak with an offset such that Ωτ = π/2 the sequence results in all of the equilibrium magnetization appearing along the x-axis. Further, if the delay is such that Ωτ = π no transverse magnetization is excited.
Use vector diagrams to explain these observations, and make a sketch graph of the amount of transverse magnetization generated as a function of the offset for a fixed delay τ.
The sequence has been used for suppressing strong solvent signals which might otherwise overwhelm the spectrum. The solvent is placed on resonance, and so is not excited; r is chosen so that the peaks of interest are excited. How does one go about choosing the value for τ?