Using the right mouse button will rev up the gun without firing. For those who aren't familiar with the term, the jump-rev is a tactic the HW uses to get around the slow spin-up time of his gun - it goes like this:
1. Jump in a direction of your choice
2. Just after your feet leave the ground, hold the right mouse button to start revving up the gun
3. By the time you hit the ground, you gun will almost be up to speed
You've travelled further in that time than you would have done if you revved the gun on the ground, which instantly slows your speed. (When you jump-rev, your speed doesn't slow until your feet hit the ground again.)
I've been watching (and re-watching, and re-watching) 4k^Riemu's HW frag vid, and from that (and my own play) here are my current practices:
When in closed areas, in close-quarters combat (e.g. Turbine, 2fort)
Jump-rev when approaching a corner which you want to shoot around, just as a way of getting there a little sooner with your gun spinning. Don't pop round the corner mid-air, aim to land just before the corner turns.
Jump-rev every time you fall into an area where there may be enemies - if there's noone there, you can unrev and hardly lose any time
Be very careful about jump-revving around a corner, rather than just up to the corner. This can end up with you stranded and in line of rockets, stickies and sentry fire with no safe retreat. However, there are cases when this is useful; often, a sol/medic will hang back and shell you round the corner; they can deal damage better than you can in that situation, and will eventually kill you with a crit. Here, you want to jump-rev past the corner, cut off and kill the soldier or medic then retreat again.
This highlights a general principle of play - use jump-rev to get into a position where an enemy has to pass you in order to escape; that way, you can repeatedly bite off and chew small pieces of the enemy team without fully exposing yourself. Jump-rev past a bad guy, then strafe back into safety while shredding them with Sasha. I do this a lot in Mach4 and Turbine when coming out of my flag area and into the enemy flag room.
If you think there's a nearby sentry, don't jump-rev, as it will launch you. If you stafe-rev, you've got a good chance of killing it.
If you think there's a distant sentry, it's sometimes useful to jump-rev; if the sentry sees you, it will push you back and out of range. However, this is very situational, and it's often better to strafe-rev here too.
Needless to say, if you have invuln. uber and you think there's a sentry, don't jump under any circumstances.
In outside areas, medium-range combat (e.g. Gravelpit, much of Granary)
As above, if you have invuln. uber and you see stickies; if you jump, the demo can launch you and break the uber connection. So just walk over these.
If you meet an enemy and you're not revved up, your usual practice should be to jump-rev and kill them. Sometimes you need to run, sometimes you need to fight - I won't discuss this here, you need to get your own feel for it.
So, when you jump-rev, some clases you want to jump towards, others away from. This can vary but here's my rule of thumb:
1. Soldiers: Jump diagonally towards. You need to be close to kill them, and they also hurt themselves at this range. However, don't jump directly at him, as you're asking for a direct hit with a rocket.
2. HWs: Jump sideways. HW-on-HW is a tracking battle where you have to centre the other HW better than they centre you. Lateral momentum is most important here.
3,4. Pyros, spies: Treat these the same way; jump away, as they can only hurt you up close.
5. Scouts: You shouldn't let a scout kill you in the open. So, jump in the direction which denies them cover, from their current position. E.g. if there's a