It almost sounds like you just decide when they come in?
via Warhammer Community
https://www.warhammer-community.com/2017/05/17/new-warhammer-40000-reserves-and-deep-striking-may17gw-homepage-post-4/
There are some pretty fundamental changes to how they work though. For a start, not every unit can be placed in reserve, so most of your army will usually deploy on the battlefield at the start of the game. This means that they will be contributing to the battle from turn one, but will also be a target for the enemy – so you’ll have to work out how to best utilise those early turns.
Quite a few units still have the option to join the game mid-battle though, and they use a variety of mystical or technological means to do so. While there are no longer universal special rules like Deep Strike or Outflank, many of these abilities will have common themes – so you can still expect units like Terminators to teleport down, Genestealer Cults units to ambush and Ork Kommandos to use their kunnin’ to sneak up on the foe.
As an example, let’s take a look at the special rule for a Trygon, a unit famed for its unusual method of deployment, tunneling up under the enemy army:
So we can see that this will be quite a powerful ability. Not only delivering the Trygon into the heart of the enemy force, but also an accompanying unit of Tyranids. And there’s nothing stopping them from charging this turn either! Though that 9″ distance to the enemy (which is common to a lot of units with similar abilities) will mean that the averages on the dice will be against you for that 2D6 charge distance. (You can always use your Command Re-roll of course…)
In matched play, there are a few additional restrictions to deployment methods like this, which you might imagine, can get very powerful very fast when used by multiple units across a single army. Matched play games use a special mission rule called Tactical Reserves.
This rule helps limit some of the more extreme cases of withholding reserves in competitive games. So, while it’s totally possible to have an all Deathwing Terminator army, for example, you can’t use the teleport rules on all of them in matched play. (Though we do think a narrative game where the entire Deathwing teleports in on the first turn to take a Chaos bastion would be pretty awesome.)
We’ll be back tomorrow with a bit more news on vehicles in the new Warhammer 40,000.