Different types of central relaxation oscillations areobserved in the presence of ECH dependingon the location of the deposited power. In the TCV tokamak, normal sawteeth, i.e. triangular sawteeth similar toohmic sawteeth, and saturated sawteeth are observed with central ECHpower deposition, while giant sawteeth and `humpback'oscillations occur when heating close to the sawtooth inversionsurface of the local soft X ray emissivity. New measurements with hightemporal resolution show that the crash phase of these sawtooth typesis accompanied by a reconnection process associated with an m/n = 1 resistiveinternal kink mode. After the sawtooth crash, full magneticreconnection is observed in normal and in saturated sawteeth, whilefor giant and humpback sawteeth the reconnection process is incompleteand poloidally asymmetric temperature profiles persist after the crash. The detailed dynamics of the magnetic island associatedwith the resistive internal kink mode are described by a displacementfunction which is inferred from the experimental data. In normalsawteeth, the kink mode is destabilized just before the crash, whilein all other sawtooth types a magnetic island exists for a significantfraction of the sawtooth period. The different types of sawteeth havebeen simulated using a numerical code based on a theoretical modelwhich describes the evolution of the electron temperature in thepresence of localized heat sources and of a magnetic m/n = 1/1 island.