Cytoplasmic tyrosine kinase - Lck in its inactive state is bound to CD4/CD8 cytoplasmic tail and it's terminal tyrosine is phosphorylated. Dephosphorylation of this amino acid (by tyrosine phosphotase CD45 -- CD4/CD8 binding its ligand) causes a conformations change in LcK and it becomes an active tyrosine kinase.
LcK is activated when the extracellular part of CD8 binds its (MHC:peptide) ligand. Lck is a Src family kinase that is constitutively expressed. It phosphorylates all TCR ITAMS.
Rephosphorylation of this carboxyl-terminal tyrosine by Csk returns Lck to the inactive state.
Basically, Lck is bound to CD8. When CD8 binds MHC:peptide, Lck gets activated and can phosphorylate nearby ITAMs.