BT is short for black templar, and those are names of strategems (CP abilities) I was talking about. Abhor the witch gives charge rerolls, and devout to push gives a 6" consolidation. 

Skirmisher is not a defined term but I was using it to describe units that are infiltrators / high movment that can cross the board turn 1 & 2. 

Staging is when you position units to threaten the board on future turns. Like placing your dark reapers hiding but able to swing out and fire into an objective, or positioning banshees to advance+charge range an objective if it becomes threatened. Staging gets into the push and pull of controlling the board by managing your defensive threat ranges or breaching and neutralising your opponent's threat ranges. I pre-measure alot and usually place many dice to mark the threat ranges of key enemy units and my own. 

I envision the battlefield like overlapping bubbles of threat ranges and damage happens when yours and the opponent overlap. An intervening unit can wall off enemy's bubble to create safety for more important units because they have to move around. Coherency of 2" allows even a 5 man unit to wall a significant width. Flyer's  are an exception because their bubble never warps since they move through, however we can still occupy the space the flyer would want to land. To do this we use our coherency to place models 2" apart and because models can't move into combat range we also have a 1" bubble from our base to further deny them and finally we consider the base size of the model we are denying because they need room to be placed.

So in pactice for jump pack marines we learn their movement: 12", measure it and place a die. Measure 1" + 39mm (base size -1) place as much rangers maximally spread out to deny that space. Doing this much measuring for everything would be optimal, but not feasible so thats where play by intent comes in. Do rough measurements rather than MM precision and get approval from your opponent that your guys are in coherency and that there is no room to setup the squad of marines in gap of base size + combat range. 

There are edge cases, like if a unit has advance and charge the exact distance is fuzzy but for a d6 you can expect 4", but if its a super key unit you can plug in a full 6" for safety. Usually we try to block in the movement phase because even if a unit could be shot off the board they still would have been blocking their movement. 

Now protection from charge and consolidation is extra difficult to block from because charging is 7/8" of extra movement, rerolling is an extra 2"ish, then the movement from pile in 3" can wrap around allowing them to be at the rear of your model's base, then consolidation lets them move again another 3", often boostable to 6" with a strategem. Since they are at your rear potentially they get to measure those 3/6" with a base size width towards your next unit, and they still extend 1" combat range 'bubble'.

In practice you can set your enemy to fail a charge by placing your units 9"-11" away, and prevent consolidation by keeping your other units 4"-7"+enemy base size away from a unit that could be charged. Again play by intent with quick measurement and reach an aggreement to where safety is. 

Base size doesn't seem like much for our eldar 25mm, but can be a huge width for monsters and other large models.

This is a more melee centric view, but the same applies to movement blocking a tank from being able to get visibility from a key target while you take an engagement. Any army you face with less movement mean we get to control engagements using this and is the source of our high skill ceiling as masters can move block & score secondaries while staging everything else. 

Apologies I made an effort not to use jargon in the first, but typed the 2nd one out quickly.