What is a Forcing Auction?
A Forcing Auction is an auction in which a side has agreed that either their side declares OR the other side plays their contract doubled.

What is a Forcing Pass?
A Forcing Pass is a Pass that FORCES partner to decide whether to bid to a higher level or double the opponents for penalty.

Study the auction above. I recommend that this be a forcing auction.
North-South have bid a vulnerable game. Neither one has shown the weakness of a pre-empting hand.
After West bids 4, North chose to make a FORCING PASS.
North's PASS suggests to South that bidding 5 might have merit.
If North didn't want to make that suggestion, North should double.
Double wouldn't necessarily promise tricks on defence. Double is the alternative to suggesting South consider bidding 5.

Why would North want to encourage South to bid 5?
The penalty of 4X might not be enough to compensate for possibly making 5(matchpoint scoring) and/or North has a hand better suited to offense.
Of course North should have a good reason for suggesting risking the higher level (including enemy suit shortness, extra trump suit length, extra values).

Here's a North hand that should consider making a Forcing Pass: x Kxxx Kxxx xxxx
It has an extra trump and shortness in the enemy suit.

Here's another Forcing Pass consideration: xxxx Axxx Kxx xx
How many enemy spades does partner have if the opponents have a nine card trump fit?
None.
North can see lousy spades that South CAN'T see.

Hands where a double (rather than a Forcing Pass) is in order:
QT98 Axxx Jxxx x (great defense)
Q98 Axx Jxxxx xx (no extra trump and some defence)
98 Kxxx Kxx xxxx (minimum making 5 unlikely)

Why double? To suggest to partner that bidding on is probably more dangerous than defending..

Consider now South's 1 opener and choices after North's forcing Pass: Double, bid 5, or make an intermediate bid.
South should double if South has: no extra values , values in the enemy suit OR no extra trump length
If South has decided to bid 5, there are useful lead-directing intermediate bids (4N:lead a trump, 5:lead a club and 5:lead a diamond) in the event the opponents compete to 5.
Bidding 5 then shows no strong interest in any of the other leads.

Here are some South hands:
543 KQJ876 AQ AQ might bid 5, expecting partner's spade shortness
32 KJT876 AKQJ5 - might bid 5 to start a possible defence of 5 with a ruff
32 AQT85 AKQJ5 9 might bid 5
AK3 KQJT86 K2 K2 might bid 4N, asking for a spade lead, to cut down on possible cross ruffing
KQJ AJT85 K2 K43 should just double for penalty

Every 2/1 Game Forcing auction is a Forcing Auction!

Vulnerable: North South Dealer:South      IMP Scoring South 1 4  X West Pass 4 Pass North 2 Pass* 5! East 3 Pass *Forcing Pass