Hand tools are used for various reason, doing stairs, working on stains and cleaning settees, chairs and suites.
The vacuum works all the time but the liquid is only supplied on demand. Simply squeeze the trigger and the mix of water and soap will come out the spray nozzle.
The spray head should give an even and wide spray delivery of the liquid. If this doesn't happen then check any filter first, this may be blocked. There also could be dirt in the liquid feed line or end nozzle itself. If you're unlucky there may even be something wrong with the pump.
The tool needs to be attached to the liquid source. This is normally a liquid reservoir on the machine, the same reservoir that is used to deliver water and soap to the carpet through the a wand or an upright cleaner.
The vacuum head is in front of the spray so activates when drawn backwards, in the same way as a wand.
If you are removing a stain then always start using plain cold water, it's surpring how often that works.