Wu Yu Xiang's Si Zi Mi Jue, or Four Word Secret Formula, and commentary:

敷：敷者，运气与己身，敷布彼劲之上，使不得动也。
Spread: Spreading means, displacing and transporting together with the qi in one's own body, spreading and distributing it strongly from above, so as to bring about the impossibility of movement.

盖：盖者，以气盖彼来处也。
Cover: Covering means, to use qi so that it covers the other's arrival and stops/ceases it.

对：对者，以气对彼来处，认定准头而去也。
Check: Checking means, to use qi so that it checks/opposes the others arrival and stops/ceases it, knowing how to focus with accuracy and then leave/depart.

吞：吞者，以气全吞而入于化也。
Swallow: Swallowing means, to use qi so that it is completely swallowed whole and then joins with / enters into transformation and neutralization.

此四字无形无声，非懂劲后，练到极精地位者，不能全知。
是以气言，能直养其气而无害，使能施于四体，四体不言而喻矣。
These four words are without form without sound, unless comprehending rulership of energy power, and practicing to arrive at the utmost point of pure essence of qi and spirit's place of existence, there is no capability to completely understand this.

So when it comes to qi, if one can nourish the qi directly without causing any harm to it, so that it can be applied to the four limbs, the four limbs do not need explanation for their use to become clear.

• Fu 敷 'Spread'

Move Qi throughout your body; use it to cover your opponent’s force so that he cannot move

Spread out and open, lay out
spread widely, disseminate, distribute, diffuse, scatter
to elaborate, expand, develop, set forth, deploy; display, exhibit, perform, express, expose, assert, present

• Gai 蓋 'Cover'

thatching a roof, put on top
canopy, covering / cover, protect, shield
uphold, respect, venerate

• Dui 對 'Check'

confront, contrapose(d); to face, be face to face; in the face of / to address, respond, reply; respondent (in a court case) / be on the opposite side, stand against; oppose antagonist(ic)
match up, form a pair, pair up; corerspond with; counterpart. / be in parallel, contraposed / relative, relativity

• Tun 吞 'Swallow'

swallow whole, gulp down / devour, consume, take in, imbibe
embrace, enclose, contain / to appropriate, annex, overpower