'Play' and 'Commit' are two different game terms. Wendy's Amulet allows you to 'play' the topmost event card in your discard pile, but it does not allow you to 'commit' it to a skill test.
The Forced effect triggers after you play an event from either your hand or your discard pile. It does not trigger after you commit an event card from hand to a skill test.
You can only have 1 copy of your investigator's signature card in your deck. These cards have no level, which is different from having level 0, and the deckbuilding rules only allow you to add cards of level 0-5 to your main deck. (Note: Signature cards in the Core Set were accidentally misprinted using the same template as level 0 cards. Starting with The Dunwich Legacy deluxe, signature cards will be printed as cards with no level (e.g. Duke).
- The Forced effect triggers after the event is played, before it is placed in your discard pile. If the event would not enter the discard pile at this time (for example an event that attaches to a location, such as Lure, or stays in play for a period of time), then this Forced effect does not trigger. In other words, Lure would be discarded as normal, because it is being discarded at a time other than 'after you play it'.
Wendy Adams deck only.
You may play the topmost event in your discard pile as if it were in your hand.
Forced - After you play an event: Place it on the bottom of your deck instead of in your discard pile.
Wendy's Amulet is a great Signature card that allows Wendy Adams to focus on an event-heavy deck. The Survivor faction has great event cards in the form of Lucky! and "Look what I found!", and Wendy's access to Rogue cards lets her play with very powerful events such as Elusive, Sneak Attack, and Backstab.
Wendy's Amulet takes up an Accessory slot, which is not a huge deal since there are not that many Accessories in the first place. Though it competes with Rabbit's Foot and the Elder Sign Amulet, it's genuinely not a big deal to overwrite those other assets with this powerful Relic.
With Wendy's Amulet in play, you can play the topmost Event in your Discard Pile as if it were in your hand (note that Play means paying the cost to play the card, you can't commit the card to a skill check!). The Forced effect of the Amulet states that playing an event requires you to place it on the bottom of your deck as opposed to your discard pile. The way these two interactions work, you generally want to play Wendy's Amulet only after you have played a couple of very powerful event cards, and then play them off the top of your discard pile.
This makes the order in which you play your events remarkably important! If you play a "Look what I found!" to Investigate and then play Wendy's Amulet, you better be sure that you want to Investigate again in the near future, or else you won't be able to dig through your discard pile for the other powerful events you just played! The importance of order makes it so playing generic events that are almost always useful, such as Emergency Cache or Will to Survive allows for greater flexibility moving forward. Of course, if you really need to focus down getting clues or evading or killing a monster or so, then you can time the Amulet well enough with those appropriate event cards.
Of course, as with all Signature cards that don't start the game in play, you can't guarantee Wendy drawing it. As such, unless it's in your hand or you have some way of ensuring you get it very soon, it's not worth fretting too much over the order in which you play your events (though, in the same breath, if you're going to play two or more events in a turn, it is worth thinking about which one you'd play last...). And, if you draw it at an inopportune time, you can always pitch it in to a skill check for the +2 Wild icons. Also, the fact that it is an Item makes it a great target for Scavenging.
An interesting part of the Forced ability is that the rules of "Instead" cause it to only place the Event card at the bottom of your deck at the point in time where it would resolve and then go into your discard pile. There are some event cards that have been revealed that do not place them in the Discard Pile but rather attach them to some location: #Lure is the example we have for now, but spoilers have revealed other cards... In the case of #Lure, after you finish playing it, it does not go straight to the Discard Pile but rather attached to your Location. As such, Wendy's Amulet Forced ability does not trigger as the "Instead" part of the text does not happen. At the end of the round, when Lure is discarded, it will go on the top of the Discard Pile (the Forced part does not trigger for this either, as it was not 'just' played). Consequently, next round, Wendy can play the Lure again from her Discard Pile! This allows Wendy to infinitely recur the Lure, and help keep monsters away from the other investigators!
EDIT: With Path to Carcosa out, Hiding Spot is now a much better card for Wendy to do some crazy shenanigans against the enemies out of the Encounter deck. With the ability to keep recurring Hiding Spot out with the Amulet, she could play it multiple times to help keep critical locations safe for their fellow teammates. This works because, like Lure, Hiding Spot does not go into the discard pile.