Image by NIKOLAUS INGENERI
It's one thing to fight them, it's another to cut them off
Not all missions allow the Marine player to secure or lock down Genestealer entry points and even those missions that do are still difficult. As the Marine player, you have to decide if spending the time and manpower to attempt to secure an entry point is worth it or if you're better off dealing with the Genestealers as they come.
How we use these two options
If we're playing to the mission specs as they're written and this option is in there, we use our version of the rules below. If we're playing a mission that doesn't have these options in it, we'll often have a quick discussion about including them along with any potential problems it may create in addition to how it might alter the game dynamics.
Sometimes just including these options is enough to give the Marine player some flexibility. He needs to choose... will he opt to forget about it and go straight for the objective or delay his attack in favor of potentially weakening the Genestealer approach.
The difference between Securing and Locking Down
Securing a Genestealer entry point is a fairly straightforward action. A Marine that is within 6 squares of a Genestealer entry point at the start of the Genestealer phase will force any blips placed at that entry point to lurk for a turn before coming onto the board.
All you're doing here is buying yourself some time. This may be all you need though to clear a difficult intersection or get past a critical choke point on the board.
Locking down a Genestealer entry point is a good bit more dangerous. To "Lock Down" a Genestealer entry point, a Marine must be able to stand in a Genestealer entry point and spend either 1AP/CP to lock it down. All Genestealers lurking at that entry point are lost and the entry point may no longer by used by the Genestealer player for the rest of the game.
In the end, it's about control
It comes down control really. As the Marine player your resources are extremely limited and the situation gets worse by the turn. Having these options available (when the mission permits) can give you the added edge you might be able to use to your advantage. The trick becomes knowing when to commit and go for the lock down and when to stay focused on the objective.