Generally, engineers in-own-base serve a defensive strategy: they repair your damaged buildings. Hereby, the main problem is, that they get crushed by enemy tanks easily - before doing their job. Even if u hide them carefully behind buildings - like in top-corners of the refinery or back-base - a skilled rusher can forced-move kill them with his tanks, or even more sophisticated: smash ´em via QC with not losing a single second of firing at your buildings.
In order to bypass these disadvantages, Break shared an old wisdom* of hiding engis IN a building - so they cannot be crushed:
1. Produce some engies and place them next to the building, in which you wanna hide them.
2. This building needs some small damage, example: shot once with a riffle at it ( it cannot be in full-health, because u glitchy use the engis´s repair-effect to bring them in).
3. Now Timing: Select engis and let them repair this building (send them in) BUT press S-Button (hotkey for stop), when they trespass the line between field and building - this actually may need some practice of timing: if u stopped them to soon, then most of the engis will halt outside; if u stopped them too late, some engies will get consumed to their repair-function.
4. If u got them stopped right in time, those engis are now hidden "inside the building". Now u can either select them singly by clicking on them or u can group them under several #numbers and use them commonly.
5. Hidden engies this way cannot be crushed by vehicles.
In this example, see the refinery with 2 silos, all full of hidden engis. Note: i was obviously too slow on down-left side: 4 instead of 5 engis = one was absorbed by repair-function.
As any infantry, u can place 5 engis in one square of the field. Thus, depending how many squares your selected building has, the many engis u can hide inside theoretically:
silo: 1 square ~ 5 engis
atom silo: 2 squares ~ 10 engis
small power: 4 squares ~ 20 engis
cy: 9 squares ~ 45 engis
1. Biggest minus is the time u need to hide them. Imagine giving a short damage with a riffle, then produce the engis, then send them and stop them rightly timed - not easy to do, when 10 other things happen in an early game as well.
2. Another minus are the costs. Imagine 1 engis costs 500$, then 5 cost 2500$ - imagine putting the theoretical 45 engis in one Cy is 22.500$ - and who, practically, has that amount of cash in early game? Lololol...
3. Most of the players using engis as defense, tend to place them scattered over their base, each next to important buildings - imagine the time u need to do this glitchy trick 2-3 times in a row with several buildings.
4. A definitly plus is: Your engis now can't any longer be crushed by vehicles.
5. Another plus can be, if u just place them in one base-central building, but use them, wherever the enemy´s attack is going against.
6. As u can clearly see in the ref-example: EVERYBODY can see those engis from far away. Haha. So one could question: How "hidden" are so much obvious engis? Meaning: If u see as a rushing attacker such a building, you would make a huge circle around it, and attack alternative structures.
7. However, if u just hide like 5-10 engis in the back/behind of a building like Cy or war factory, one cannot any longer see so clearly the now more-hidden engis.
Test, experimentate and share your results and comments below.
*U can find this hint merely in JC´s guide by mentioning it´s basic function in one line text, but not in milo´s.