its usually half a litre liquid in a can. pour it in engine oil just before oil change. i prefer to drive the car around for 10-20 mins, near the end, try to take it to higher rpms 2-3 times.
It cleans out the gunk from valvetrain (engine head) and underside of pistons - basically everywhere engine oil goes and might leave deposits. Use it esp. if experiencing sticky valves or if you've been using inferior grade engine oils/ mineral oils and want to shift over to semi-synth or synthetic oils.
diesel is a dirtier fuel as compared to petrol so it might leave more deposits, engine flush can sometimes cure a diesel car's smoky behaviour.
Ofcourse drivability usually gets better.