Firstly, you need to be aware that Kew Alto/Nilfisk pressure washer accessories have two types of fitting, a sturdy bayonet fitting with O-ring ie "professional grade" , and a cheaper plastic fitting without any O-ring known as Click & Clean ie "consumer grade". Accessories are made with either fitting, and are supplied with an adapter to convert one to the other as appropriate.
The Underchassis Lance is a "professional" quality item with a brass bayonet fitting, and represents excellent value. It is designed for use on vehicles, and so has a spray pattern which will not damage the underseal or paintwork. It is ideal for removing salt and mud from the wheelarches and underbody, or reaching the sides and/or roof of taller vehicles. It is also ideal for cleaning building gutters.
The Gun and Extension Lance supplied with consumer grade "Compact" Nilfisk Pressure washers, such as C120.3, C120.5 and C125.3, are a mixture of the two fittings. The gun ends with a female bayonet fitting, so that any "professional" grade accessories can be directly attached. The extension lance has a male bayonet to fit the gun but ends with a female Click & Clean fitting, to accept all the "consumer" grade accessories.
The Underchassis Lance bayonet fits directly into the gun, and is convenient to use in this configuration. You need 2 hands and the lance can be twisted to the desired angle with the forward hand as you work. However, in this configuration it is the same length as the Click & Clean Underchassis Nozzle when fitted into the extension lance. The Underchassis Nozzle can be used in much the same way as the Underchassis Lance, and seems to have a slightly more aggressive spray pattern, so why would you need both?
Well, some consumer accessory kits, notably the Water Suction Kit and Patio Cleaner Plus, come with an adapter with female bayonet and male Click & Clean. This can work both ways. Using this adapter, the Underchassis Lance can be fitted on the end of the Extension Lance to provide an extra long reach, which can sometimes be very useful.
I have seen the comments about lack of aggressive cleaning power, but if you really want to remove the underseal from your vehicle with a powerful spray jet, then you need the right angle adapter instead, which you can then use with the supplied Tornado or Powerspeed nozzles designed for maximum cleaning power.
All in all, I am very pleased with the underchassis lance as a useful addition to the other accessories available from Nilfisk.