G' Day Alejandro,
Let's see if this helps:
When a square rigged ship turns into the wind up to the "Point" flat edge portion of the "S" segment of the Wind Gauge, she can then expend remaining MFs sailing along that course (as close to the wind as she can "point" - have her sails draw) or she can continue to turn into the wind. If so, a square rigger has two alternatives:
If she continues to turn into the wind with less than 4 MFs left to expend, she comes to a halt when "head to wind" (bow pointed into the wind) and is "in irons." She stalls out, comes to a stop and starts drifting backwards for four Tactical Phases while her crew regains control and gets her moving again as described in the "In Irons" bullet at the top of page 1-9. Thus a captain would not want to continue turning into the wind if his vessel has less than 4 MFs left to expend (i.e., does not have enough momentum [4 MFs] to tack).
If she does have 4 or more MFs left to expend, she has enough momentum to tack. That is done as a four Tactical Phase maneuver as detailed in rule Section 4.6 (page 1-8). Briefly, this is done as follows:
Phase 1 - Sail a "pointing" course and then turn head to wind, expending 4 MFs - and any remaining excess. As the ship turns bow into the wind she loses thrust and the wind blowing on her hull and rigging acts a a brake, bringing her almost to a stop. The four or more MFs are required to give her the needed momentum to carry her bow around far enough through the wind for her jibs to begin to draw on the other tack.
Phase 2 - In Tactical Phase 2, her captain casts a D12 and consults the TACKING table on Chart 2 B while the vessel remains head to wind. This simulates whether or not the crew's timing in shifting the jibs and other sails is effective as the ship loses momentum and her turn slows. The key is timing. If sails are shifted too soon, they will stall the vessel; if too late, the ship comes to a stop before the jibs can help drag her bow far enough around to complete the tack. Thus, shifting the sails must be done in careful coordination with the slowing, turning hull. The more experience, the better a crew is at this maneuver. That is why the crew's quality is a key determinant on the TACKING table.
Phase 3 - If successful, shift the vessel over to the new tack by pivoting her by the stern (or aft end of her base). This simulates that she is essentially stopped while her bow is forced around to the "Point" course for the new tack. All this, takes time. Hence, multiple Phases are required.
Phase 4 - The ship begins to sail forward on the new tack by expending 2 MFs. Thereafter, she can accelerate normally.
All this may seem complex at first, but it is a continuous process that is easily understood after you perform it once or twice. Two other clarifications are also in order:
1. MFs cannot be stored or carried over to the next Tactical Phase. A ship must expend all her MFs each Tactical Phase (rule Section 4.2) or decelerate as described in rule Section 4.4. When turning into the wind, all remaining MFs are quickly drained away as a vessel loses sail thrust and the wind resistance against her hull, rigging and sails increases.
2. The tacking process is similar, but much easier for a fore & aft rig, such as we sailors use in modern times. Fore & aft sails provide thrust significantly closer to the wind (approximately 45°) than square rig sails (more than 60°) and modern hulls are smaller and lighter, providing more momentum and shorter turning times across the wind. This is reflected in the Fore & Aft Wind Gauge. It only requires 3 MFs to tack and only three Tactical Phases are needed to complete a tack as described in rule Section 4.6 at the bottom of page 1-8. The probability of success is also much higher than a square rigger on the TACKING table.
Hopefully, this clears up the maneuver options for you. If not, we can discuss further,