Briefing in engineers is bringing them up to speed on the project at hand, and breaking off a relevant piece that they achieve. This has a fixed cost, in that the startup of an engineer on a job takes time and effort. At the very least, there will be more people at your kickoff meeting.
If you have a very comprehensive engineering process, then the startup will not be too expensive. In any event, adding an engineer adds a communication layer, which will require capture and negotiation. So more engineers in parallel is more expensive, with all other factors held constant.
So to do a 12 man month job in 4 months, you will have to brief in 4-6 engineers and break out the work for them. Why so many? Because, probably, one of them will not work out due to availability, skill, etc. If the effort is not well understood, accept the cost to brief in another.
Of course, if you have true experts, the brief in can be very small. But briefing in more engineers costs more. And you may not need them.
Many times, this briefing in is done for new grads or junior engineers to prepare them for the senior partner moving off. It is natural and, if done correctly, will allow the senior partner to move off the job. This may provide an overall budget improvement, especially if you have the schedule to allow the growth of the individual.