Ions spend most of their time at the turning points (in ETD region), which facilitates cation-anion interaction.

Relative velocity approaches zero, necessary for efficient ETD or ECD.

Fragment ions are born with the same potential energy as parent ions.

KE of the cations and anions at the turning points in negligible. This prevents collision induced dissociation and also provides necessary initial conditions for the ETD (or VUV photodissociation) ion fragments to remain trapped and, hence, can be accurately detected with pickup electrode or TOF imaging detector.